From 4d583eb3abf482e02061f72cbf3f3d75d8b51479 Mon Sep 17 00:00:00 2001
From: Cristiano Piemontese <cristiano.piemontese@prima.it>
Date: Fri, 6 Feb 2026 12:21:19 +0100
Subject: [PATCH] specialize validation for metric and event_name

---
 lib/telemetry_metrics.ex        | 29 ++++++++++++++++++-----------
 test/telemetry_metrics_test.exs | 12 +++++++-----
 2 files changed, 25 insertions(+), 16 deletions(-)

diff --git a/lib/telemetry_metrics.ex b/lib/telemetry_metrics.ex
index f4b32d4..9b63252 100644
--- a/lib/telemetry_metrics.ex
+++ b/lib/telemetry_metrics.ex
@@ -557,7 +557,7 @@ defmodule Telemetry.Metrics do
 
   @spec common_fields(metric_name(), [metric_option() | {atom(), term()}]) :: map()
   defp common_fields(metric_name, options) do
-    metric_name = validate_metric_or_event_name!(metric_name)
+    metric_name = validate_metric_or_event_name!(metric_name, :metric)
     {event_name, [measurement]} = Enum.split(metric_name, -1)
     {event_name, options} = Keyword.pop(options, :event_name, event_name)
     {measurement, options} = Keyword.pop(options, :measurement, measurement)
@@ -566,7 +566,7 @@ defmodule Telemetry.Metrics do
     validate_recording_rule_fun_options!(keep, drop)
     keep = validate_recording_rule_fun!(keep)
     drop = validate_recording_rule_fun!(drop)
-    event_name = validate_metric_or_event_name!(event_name)
+    event_name = validate_metric_or_event_name!(event_name, :event_name)
     {unit, options} = Keyword.pop(options, :unit, :unit)
     {unit, conversion_ratio} = validate_unit!(unit)
     measurement = maybe_convert_measurement(measurement, conversion_ratio)
@@ -584,38 +584,45 @@ defmodule Telemetry.Metrics do
     })
   end
 
-  @spec validate_metric_or_event_name!(term()) :: [atom(), ...]
-  defp validate_metric_or_event_name!(metric_or_event_name)
+  @spec validate_metric_or_event_name!(term(), atom()) :: [atom(), ...]
+  defp validate_metric_or_event_name!(metric_or_event_name, :metric)
        when metric_or_event_name == [] or metric_or_event_name == "" do
-    raise ArgumentError, "metric or event name can't be empty"
+    raise ArgumentError, "metric can't be empty"
   end
 
-  defp validate_metric_or_event_name!(metric_or_event_name) when is_list(metric_or_event_name) do
+  defp validate_metric_or_event_name!(metric_or_event_name, :event_name)
+       when metric_or_event_name == [] or metric_or_event_name == "" do
+    raise ArgumentError,
+          "event_name can't be empty (metric must be namespaced or :event_name explicitly set)"
+  end
+
+  defp validate_metric_or_event_name!(metric_or_event_name, kind)
+       when is_list(metric_or_event_name) do
     if Enum.all?(metric_or_event_name, &is_atom/1) do
       metric_or_event_name
     else
       raise ArgumentError,
-            "expected metric or event name to be a list of atoms or a string, " <>
+            "expected #{kind} to be a list of atoms or a string, " <>
               "got #{inspect(metric_or_event_name)}"
     end
   end
 
-  defp validate_metric_or_event_name!(metric_or_event_name)
+  defp validate_metric_or_event_name!(metric_or_event_name, kind)
        when is_binary(metric_or_event_name) do
     segments = String.split(metric_or_event_name, ".")
 
     if Enum.any?(segments, &(&1 == "")) do
       raise ArgumentError,
-            "metric or event name #{metric_or_event_name} contains leading, " <>
+            "#{kind} #{metric_or_event_name} contains leading, " <>
               "trailing or consecutive dots"
     end
 
     Enum.map(segments, &String.to_atom/1)
   end
 
-  defp validate_metric_or_event_name!(term) do
+  defp validate_metric_or_event_name!(term, kind) do
     raise ArgumentError,
-          "expected metric name to be a string or a list of atoms, got #{inspect(term)}"
+          "expected #{kind} to be a string or a list of atoms, got #{inspect(term)}"
   end
 
   defp validate_recording_rule_fun!(nil), do: nil
diff --git a/test/telemetry_metrics_test.exs b/test/telemetry_metrics_test.exs
index 9e859c8..7fa993f 100644
--- a/test/telemetry_metrics_test.exs
+++ b/test/telemetry_metrics_test.exs
@@ -299,11 +299,13 @@ defmodule Telemetry.MetricsTest do
       end
 
       test "raises when event name derived from metric name is empty" do
-        assert_raise ArgumentError, fn ->
-          apply(Metrics, unquote(metric_type), [
-            "latency"
-          ])
-        end
+        assert_raise ArgumentError,
+                     "event_name can't be empty (metric must be namespaced or :event_name explicitly set)",
+                     fn ->
+                       apply(Metrics, unquote(metric_type), [
+                         "latency"
+                       ])
+                     end
       end
 
       test "raises when event name is empty" do