[exporterhelper] Create protobuf schema for RequestContext and store in persistent queue

[jackgopack4]

Component(s)

exporter/exporterhelper

Describe the issue you're reporting

If #12934 is landed, this will store spanContext for internal batch tracking span/telemetry as JSON in persistent storage.

But since we have added internal protobuf schema for persistent queue in #13067, there should be ability to define this SpanContext detail in an internal protobuf for increased efficiency in operations.

Alternatively, there could be the ability to save this alongside requests, if the request protobuf schema adds fixed or variable-length byte storage (e.g. marshal SpanContext to internal persistentqueue protobuf, store in pre-allocated byte array in telemetry request).

This should be taken up as follow-up after #12934 is merged; I will be happy to take it up. exporter.PersistRequestContext is in Alpha and this behavior will be behind the featuregate, so even if it were to be a breaking change (which we could mitigate with maintaining fallback logic for a certain number of versions), it would be allowed.

[github-actions]

Pinging code owners:

• exporter/exporterhelper: @bogdandrutu @dmitryax

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