Allowing tags to start with _ (#5478)

* Allowing namespaces to start with _

Making it possible for namespaces to start with _. This is necessary to use OT exporter as it requires namespace overrides to start with _

* Adding test that checks if a tag can start with _

src/Generators/Microsoft.Gen.Metrics/Parser.cs

@@ -22,7 +22,7 @@ internal sealed class Parser
     private const int MaxTagNames = 30;
 
     private static readonly Regex _regex = new("^[A-Z]+[A-za-z0-9]*$", RegexOptions.Compiled);
-    private static readonly Regex _regexTagNames = new("^[A-Za-z]+[A-Za-z0-9_.:-]*$", RegexOptions.Compiled);
+    private static readonly Regex _regexTagNames = new("^[A-Za-z_]+[A-Za-z0-9_.:-]*$", RegexOptions.Compiled);
     private static readonly SymbolDisplayFormat _typeSymbolFormat =
         SymbolDisplayFormat.FullyQualifiedFormat.WithMiscellaneousOptions(
             SymbolDisplayMiscellaneousOptions.IncludeNullableReferenceTypeModifier);

test/Generators/Microsoft.Gen.Metrics/Unit/ParserTests.cs

@@ -144,7 +144,7 @@ public async Task ValidDimensionsKeyNames()
         var d = await RunGenerator(@"
             partial class C
             {
-                [Counter(""Env.Name"", ""clustr:region"", ""Req_Name"", ""Req-Status"")]
+                [Counter(""Env.Name"", ""clustr:region"", ""Req_Name"", ""Req-Status"", ""_microsoft_metrics_namespace"")]
                 static partial TestCounter CreateMetricName(Meter meter, string env, string region);
             }");