Limit character set according to spec (#3821)

Author: mic-max

src/OpenTelemetry/CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+* Fix instrument naming enforcement implementation to match the spec.
+  ([#3821](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3821))
+
 * Added support for loading environment variables from `IConfiguration` when
   using the `MetricReaderOptions` & `BatchExportActivityProcessorOptions`
   classes.

src/OpenTelemetry/Metrics/Builder/MeterProviderBuilderSdk.cs

@@ -24,7 +24,7 @@ namespace OpenTelemetry.Metrics
     internal sealed class MeterProviderBuilderSdk : MeterProviderBuilderBase
     {
         private static readonly Regex InstrumentNameRegex = new(
-            @"^[a-zA-Z][-.\w]{0,62}$", RegexOptions.IgnoreCase | RegexOptions.Compiled);
+            @"^[a-z][a-z0-9-._]{0,62}$", RegexOptions.IgnoreCase | RegexOptions.Compiled);
 
         public MeterProviderBuilderSdk()
         {

test/OpenTelemetry.Tests/Metrics/MetricTestData.cs

@@ -28,6 +28,7 @@ public static IEnumerable<object[]> InvalidInstrumentNames
                     new object[] { "1first-char-not-alphabetic" },
                     new object[] { "invalid+separator" },
                     new object[] { new string('m', 64) },
+                    new object[] { "a\xb5" }, // `\xb5` is the Micro character
            };
 
         public static IEnumerable<object[]> ValidInstrumentNames
@@ -39,6 +40,7 @@ public static IEnumerable<object[]> ValidInstrumentNames
                     new object[] { "my.metric" },
                     new object[] { "my_metric2" },
                     new object[] { new string('m', 63) },
+                    new object[] { "CaSe-InSeNsItIvE" },
            };
 
         public static IEnumerable<object[]> InvalidHistogramBoundaries