prometheus-erl · fen-pl · Apr 23, 2026
diff --git a/README.md b/README.md
@@ -164,6 +164,36 @@ Common options are:
 
 Histogram also accepts `buckets` option. Please refer to respective modules docs for the more information.
 
+#### Default series
+
+Metrics with no labels automatically get a zero-value series created at declaration time.
+Metrics **with** labels do **not** create any series automatically — a series only appears after the
+first write (e.g. `inc`, `observe`, `set`).
+
+If you need a labeled series to be present in the output before the first write, call
+`prometheus_metric:set_default(MetricModule, Registry, Name, LabelValues)` explicitly:
+
+```erlang
+%% Declare a labeled counter
+prometheus_counter:declare([{name, http_requests_total}, {labels, [method]}, {help, ""}]),
+%% Pre-seed specific label combinations so they appear with value 0 immediately
+prometheus_metric:set_default(prometheus_counter, default, http_requests_total, [get]),
+prometheus_metric:set_default(prometheus_counter, default, http_requests_total, [post]).
+```
+
+If you are using the default registry, you can use the shorter
+`prometheus_metric:set_default(MetricModule, Name, LabelValues)` form:
+
+```erlang
+%% Equivalent to the previous example, but defaults Registry to `default`
+prometheus_metric:set_default(prometheus_counter, http_requests_total, [get]),
+prometheus_metric:set_default(prometheus_counter, http_requests_total, [post]).
+```
+
+Both `set_default/3` and `set_default/4` are idempotent: calling them again for an already-created series is a no-op.
+It raises `{unknown_metric, Registry, Name}` if the metric has not been declared, and
+`{invalid_metric_arity, Present, Expected}` if the label value count does not match.
+
 ### Exposition Formats
 
 - [`prometheus_text_format`](https://github.com/deadtrickster/prometheus.erl/blob/master/doc/prometheus_text_format.md) - renders metrics for a given registry (default is `default`) in text format;

diff --git a/src/metrics/prometheus_boolean.erl b/src/metrics/prometheus_boolean.erl
@@ -41,7 +41,7 @@ fuse_event(Fuse, Event) ->
     declare/1,
     deregister/1,
     deregister/2,
-    set_default/2,
+    set_default/3,
     set/2,
     set/3,
     set/4,
@@ -127,10 +127,22 @@ deregister(Registry, Name) ->
     NumDeleted = ets:select_delete(?TABLE, deregister_select(Registry, Name)),
     {MFR, NumDeleted > 0}.
 
-?DOC(false).
--spec set_default(prometheus_registry:registry(), prometheus_metric:name()) -> ok.
-set_default(Registry, Name) ->
-    set(Registry, Name, [], undefined).
+?DOC("""
+Pre-seeds a boolean series for `Registry`, `Name` and `LabelValues` with the value `undefined`,
+if the series does not yet exist.
+
+Raises:
+
+* `{unknown_metric, Registry, Name}` error if boolean with name `Name` can't be found in `Registry`.
+* `{invalid_metric_arity, Present, Expected}` error if labels count mismatch.
+""").
+-spec set_default(
+    Registry :: prometheus_registry:registry(),
+    Name :: prometheus_metric:name(),
+    LabelValues :: prometheus_metric:label_values()
+) -> ok.
+set_default(Registry, Name, LabelValues) ->
+    set(Registry, Name, LabelValues, undefined).
 
 ?DOC(#{equiv => set(default, Name, [], Value)}).
 -spec set(prometheus_metric:name(), prometheus:prometheus_boolean()) -> ok.

diff --git a/src/metrics/prometheus_counter.erl b/src/metrics/prometheus_counter.erl
@@ -58,7 +58,7 @@ inc(Caller) ->
     declare/1,
     deregister/1,
     deregister/2,
-    set_default/2,
+    set_default/3,
     inc/1,
     inc/2,
     inc/3,
@@ -146,10 +146,25 @@ deregister(Registry, Name) ->
     NumDeleted = ets:select_delete(?TABLE, deregister_select(Registry, Name)),
     {MFR, NumDeleted > 0}.
 
-?DOC(false).
--spec set_default(prometheus_registry:registry(), prometheus_metric:name()) -> boolean().
-set_default(Registry, Name) ->
-    ets:insert_new(?TABLE, {key(Registry, Name, []), 0, 0}).
+?DOC("""
+Pre-seeds a counter series for `Registry`, `Name` and `LabelValues` with an initial value of 0,
+if the series does not yet exist.
+
+Useful for ensuring a labeled series is present in output before any increments happen.
+
+Raises:
+
+* `{unknown_metric, Registry, Name}` error if counter with name `Name` can't be found in `Registry`.
+* `{invalid_metric_arity, Present, Expected}` error if labels count mismatch.
+""").
+-spec set_default(
+    Registry :: prometheus_registry:registry(),
+    Name :: prometheus_metric:name(),
+    LabelValues :: prometheus_metric:label_values()
+) -> boolean().
+set_default(Registry, Name, LabelValues) ->
+    prometheus_metric:check_mf_exists(?TABLE, Registry, Name, LabelValues),
+    ets:insert_new(?TABLE, {key(Registry, Name, LabelValues), 0, 0}).
 
 ?DOC(#{equiv => inc(default, Name, [], 1)}).
 -spec inc(prometheus_metric:name()) -> ok.

diff --git a/src/metrics/prometheus_gauge.erl b/src/metrics/prometheus_gauge.erl
@@ -54,7 +54,7 @@ track_checked_out_sockets(CheckoutFun) ->
     declare/1,
     deregister/1,
     deregister/2,
-    set_default/2,
+    set_default/3,
     set/2,
     set/3,
     set/4,
@@ -161,10 +161,25 @@ deregister(Registry, Name) ->
     NumDeleted = ets:select_delete(?TABLE, deregister_select(Registry, Name)),
     {MFR, NumDeleted > 0}.
 
-?DOC(false).
--spec set_default(prometheus_registry:registry(), prometheus_metric:name()) -> boolean().
-set_default(Registry, Name) ->
-    ets:insert_new(?TABLE, {{Registry, Name, []}, 0, 0}).
+?DOC("""
+Pre-seeds a gauge series for `Registry`, `Name` and `LabelValues` with an initial value of 0,
+if the series does not yet exist.
+
+Useful for ensuring a labeled series is present in output before any updates happen.
+
+Raises:
+
+* `{unknown_metric, Registry, Name}` error if gauge with name `Name` can't be found in `Registry`.
+* `{invalid_metric_arity, Present, Expected}` error if labels count mismatch.
+""").
+-spec set_default(
+    Registry :: prometheus_registry:registry(),
+    Name :: prometheus_metric:name(),
+    LabelValues :: prometheus_metric:label_values()
+) -> boolean().
+set_default(Registry, Name, LabelValues) ->
+    prometheus_metric:check_mf_exists(?TABLE, Registry, Name, LabelValues),
+    ets:insert_new(?TABLE, {{Registry, Name, LabelValues}, 0, 0}).
 
 ?DOC(#{equiv => set(default, Name, [], Value)}).
 -spec set(prometheus_metric:name(), number()) -> ok.

diff --git a/src/metrics/prometheus_histogram.erl b/src/metrics/prometheus_histogram.erl
@@ -52,7 +52,7 @@ the number of time ticks when the recent value was observed).
     declare/1,
     deregister/1,
     deregister/2,
-    set_default/2,
+    set_default/3,
     observe/2,
     observe/3,
     observe/4,
@@ -178,10 +178,24 @@ deregister(Registry, Name) ->
         _:_ -> {false, false}
     end.
 
-?DOC(false).
--spec set_default(prometheus_registry:registry(), prometheus_metric:name()) -> boolean().
-set_default(Registry, Name) ->
-    insert_placeholders(Registry, Name, []).
+?DOC("""
+Pre-seeds a histogram series for `Registry`, `Name` and `LabelValues` with zero-count buckets
+and zero sum, if the series does not yet exist.
+
+Useful for ensuring a labeled series is present in output before any observations happen.
+
+Raises:
+
+* `{unknown_metric, Registry, Name}` error if histogram with name `Name` can't be found in `Registry`.
+* `{invalid_metric_arity, Present, Expected}` error if labels count mismatch.
+""").
+-spec set_default(
+    Registry :: prometheus_registry:registry(),
+    Name :: prometheus_metric:name(),
+    LabelValues :: prometheus_metric:label_values()
+) -> boolean().
+set_default(Registry, Name, LabelValues) ->
+    insert_placeholders(Registry, Name, LabelValues).
 
 ?DOC(#{equiv => observe(default, Name, [], Value)}).
 -spec observe(prometheus_metric:name(), number()) -> ok.

diff --git a/src/metrics/prometheus_quantile_summary.erl b/src/metrics/prometheus_quantile_summary.erl
@@ -54,7 +54,7 @@ It takes `error` and `bound` as in `t:ddskerl_ets:opts/0`.
 -export([
     new/1,
     declare/1,
-    set_default/2,
+    set_default/3,
     deregister/1,
     deregister/2,
     observe/2,
@@ -122,11 +122,23 @@ declare(Spec) ->
     Spec1 = validate_summary_spec(Spec),
     prometheus_metric:insert_mf(?TABLE, ?MODULE, Spec1).
 
-?DOC(false).
--spec set_default(prometheus_registry:registry(), prometheus_metric:name()) -> boolean().
-set_default(Registry, Name) ->
-    #{error := Error, bound := Bound} = get_configuration(Registry, Name),
-    Key = key(Registry, Name, []),
+?DOC("""
+Pre-seeds a quantile summary series for `Registry`, `Name` and `LabelValues` with an empty
+initial state, if the series does not yet exist.
+
+Raises:
+
+* `{unknown_metric, Registry, Name}` error if summary with name `Name` can't be found in `Registry`.
+* `{invalid_metric_arity, Present, Expected}` error if labels count mismatch.
+""").
+-spec set_default(
+    Registry :: prometheus_registry:registry(),
+    Name :: prometheus_metric:name(),
+    LabelValues :: prometheus_metric:label_values()
+) -> boolean().
+set_default(Registry, Name, LabelValues) ->
+    #{error := Error, bound := Bound} = get_configuration(Registry, Name, LabelValues),
+    Key = key(Registry, Name, LabelValues),
     ddskerl_ets:new(?TABLE, Key, Error, Bound).
 
 ?DOC(#{equiv => deregister(default, Name)}).
@@ -472,8 +484,8 @@ insert_metric(Registry, Name, LabelValues, Key) ->
     Configuration = Configuration0#{name => Key},
     ddskerl_ets:new(Configuration).
 
-get_configuration(Registry, Name) ->
-    MF = prometheus_metric:check_mf_exists(?TABLE, Registry, Name),
+get_configuration(Registry, Name, LabelValues) ->
+    MF = prometheus_metric:check_mf_exists(?TABLE, Registry, Name, LabelValues),
     prometheus_metric:mf_data(MF).
 
 key(Registry, Name, LabelValues) ->

diff --git a/src/metrics/prometheus_summary.erl b/src/metrics/prometheus_summary.erl
@@ -44,7 +44,7 @@ observe_response(Size) ->
     declare/1,
     deregister/1,
     deregister/2,
-    set_default/2,
+    set_default/3,
     observe/2,
     observe/3,
     observe/4,
@@ -133,10 +133,25 @@ deregister(Registry, Name) ->
     NumDeleted = ets:select_delete(?TABLE, deregister_select(Registry, Name)),
     {MFR, NumDeleted > 0}.
 
-?DOC(false).
--spec set_default(prometheus_registry:registry(), prometheus_metric:name()) -> boolean().
-set_default(Registry, Name) ->
-    ets:insert_new(?TABLE, {key(Registry, Name, []), 0, 0, 0}).
+?DOC("""
+Pre-seeds a summary series for `Registry`, `Name` and `LabelValues` with zero count and zero sum,
+if the series does not yet exist.
+
+Useful for ensuring a labeled series is present in output before any observations happen.
+
+Raises:
+
+* `{unknown_metric, Registry, Name}` error if summary with name `Name` can't be found in `Registry`.
+* `{invalid_metric_arity, Present, Expected}` error if labels count mismatch.
+""").
+-spec set_default(
+    Registry :: prometheus_registry:registry(),
+    Name :: prometheus_metric:name(),
+    LabelValues :: prometheus_metric:label_values()
+) -> boolean().
+set_default(Registry, Name, LabelValues) ->
+    prometheus_metric:check_mf_exists(?TABLE, Registry, Name, LabelValues),
+    ets:insert_new(?TABLE, {key(Registry, Name, LabelValues), 0, 0, 0}).
 
 ?DOC(#{equiv => observe(default, Name, [], Value)}).
 -spec observe(prometheus_metric:name(), number()) -> ok.

diff --git a/src/prometheus_metric.erl b/src/prometheus_metric.erl
@@ -17,6 +17,8 @@ as well as handling metric labels and data.
 -export([
     insert_new_mf/3,
     insert_mf/3,
+    set_default/3,
+    set_default/4,
     deregister_mf/2,
     deregister_mf/3,
     check_mf_exists/3,
@@ -85,10 +87,11 @@ as well as handling metric labels and data.
 ?DOC("Inserts a new metric function into the table.").
 -callback declare(Spec :: spec()) -> boolean().
 
-?DOC("Sets the default metric function for the module.").
--callback set_default(Registry, Name) -> dynamic() when
-    Registry :: prometheus_registry:registry(),
-    Name :: name().
+?DOC("Sets the default (zero/initial) state for the metric series identified by Registry, Name and LabelValues.").
+-callback set_default(Registry :: prometheus_registry:registry(),
+                      Name :: name(),
+                      LabelValues :: label_values()) ->
+    dynamic().
 
 ?DOC("Removes a metric function by name.").
 -callback remove(Name :: name()) -> boolean() | no_return().
@@ -159,6 +162,20 @@ insert_mf(Table, Module, Spec) ->
             false
     end.
 
+?DOC("Calls Module:set_default(default, Name, LabelValues). ").
+-spec set_default(Module :: module(), Name :: name(), LabelValues :: label_values()) -> dynamic().
+set_default(Module, Name, LabelValues) ->
+    set_default(Module, default, Name, LabelValues).
+
+?DOC("Calls Module:set_default(Registry, Name, LabelValues). ").
+-spec set_default(Module :: module(),
+                  Registry :: prometheus_registry:registry(),
+                  Name :: name(),
+                  LabelValues :: label_values()) ->
+    dynamic().
+set_default(Module, Registry, Name, LabelValues) ->
+    Module:set_default(Registry, Name, LabelValues).
+
 ?DOC(false).
 -spec deregister_mf(Table, Registry) -> boolean() | no_return() when
     Table :: atom(),
@@ -264,7 +281,7 @@ normalize_mf_row([Name, {Labels, Help}, C, D, Data]) ->
     Name :: name(),
     Labels :: list().
 maybe_set_default(Module, Registry, Name, []) ->
-    Module:set_default(Registry, Name);
+    set_default(Module, Registry, Name, []);
 maybe_set_default(_, _, _, _) ->
     ok.
 

diff --git a/test/eunit/metric/prometheus_boolean_tests.erl b/test/eunit/metric/prometheus_boolean_tests.erl
@@ -16,7 +16,8 @@ prometheus_format_test_() ->
         fun test_collector1/1,
         fun test_collector2/1,
         fun test_collector3/1,
-        fun test_values/1
+        fun test_values/1,
+        fun test_set_default_with_labels/1
     ]}.
 
 test_registration(_) ->
@@ -227,6 +228,31 @@ test_values(_) ->
         )
     ].
 
+test_set_default_with_labels(_) ->
+    prometheus_boolean:new([
+        {name, fuse_state},
+        {labels, [name]},
+        {help, "Fuse state"}
+    ]),
+    %% Before seeding: labeled series is undefined
+    Undef = prometheus_boolean:value(fuse_state, [myapp]),
+    %% Seed the labeled series
+    prometheus_boolean:set_default(default, fuse_state, [myapp]),
+    %% After seeding: labeled series returns undefined (boolean default)
+    Seeded = prometheus_boolean:value(fuse_state, [myapp]),
+    [
+        ?_assertEqual(undefined, Undef),
+        ?_assertEqual(undefined, Seeded),
+        ?_assertError(
+            {unknown_metric, default, unknown_boolean},
+            prometheus_boolean:set_default(default, unknown_boolean, [myapp])
+        ),
+        ?_assertError(
+            {invalid_metric_arity, 2, 1},
+            prometheus_boolean:set_default(default, fuse_state, [myapp, extra])
+        )
+    ].
+
 test_collector1(_) ->
     prometheus_boolean:new([
         {name, simple_boolean},