Add OpCode for activity started and stop in OpenTelemetry-Sdk eventsource

[xiaomi7732]

Hi maintainers,

OpenTelemetry-Sdk provides activity started / stopped events. How about adding OpCode on those events, making it easier to be discovered by the analysis tools?

Changes

Add opcode for activity start / stop to OpenTelemetry-Sdk event source.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: xiaomi7732 / name: Saar Shen (259b53b, 9981d61)

[rajkumar-rangaraj]

@xiaomi7732 Could you please check if you reviewed the previous PR #5387, where the feedback was provided for similar proposal?

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[xiaomi7732]

@rajkumar-rangaraj Thanks for bring that up. I didn't realize there's already a discussion like that. Reading through the comments there, I am now leaning toward thinking it not a good idea to depend on internal event source.