Escape delimiters in metric event tags

[hoyosjs]

Fixes dotnet/diagnostics#5935

Escapes backslashes, commas, and equals signs in metric tag keys and values emitted through MetricsEventSource. Event versions are incremented so version-aware consumers can distinguish the escaped representation, and coverage validates the encoding and emitted payload.

Compatibility: Existing tools continue to work for tags without these characters. Older tools do not understand the escaped representation from a newer runtime: commas can be treated as tag separators and may break legacy dotnet-counters rendering, equals signs can be truncated or misparsed, and backslashes can appear doubled. Updated tooling should use the event version to decode tags.

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Pull request overview

Updates System.Diagnostics.DiagnosticSource metric event payloads so the flattened tags strings (key=value,key=value) are unambiguous when tag keys/values contain delimiter characters. This is done by escaping \, ,, and = in keys/values, and by bumping event versions for tag-carrying events so version-aware consumers can distinguish the new encoding.

Changes:

• Escape \, ,, and = in Helpers.FormatTags for both IEnumerable<KeyValuePair<string, object?>> and KeyValuePair<string, string>[] inputs.
• Increment MetricsEventSource event versions for all events that emit flattened tag strings (including instrument/meter tags).
• Add focused unit coverage for the encoding + a targeted end-to-end EventSource test validating the emitted escaped payload.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Helpers.cs	Implements delimiter escaping in FormatTags via a shared AppendEscaped helper.
src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Metrics/MetricsEventSource.cs	Bumps event versions for tag-emitting events to distinguish escaped representation.
src/libraries/System.Diagnostics.DiagnosticSource/tests/MetricOuterLoopTests/MetricEventSourceTests.cs	Adds unit + end-to-end tests validating escaping and emitted EventSource payload tags.

[noahfalk]

Please update the PR description to link to the bug being fixed. I assume it is supposed to reference dotnet/diagnostics#5935?

heads up @tarekgh

[noahfalk]

Escaping '\' as "\\" would be a breaking change on something that I assume currently works? I'd prefer we pick an escaping strategy that doesn't introduce new failures on existing tools. For example:

',' -> ",," 
'=' -> "=="

Admitedly its a little weird but I don't imagine there are going to be that many parsers for this event data in the world.

[hoyosjs]

agreed that since this scheme is not enough and this is close to platform complete - we'll park this until .NET 12.