Specify version of the proto that introduced a field in the comments

[tigrannajaryan]

The spec uses labels like since 1.23.0 to indicate which version added the particular capability.

I wonder if we want to use a similar labeling in the form of comments when new capabilities are added to OTLP proto. This would make it easier for OTLP implementors to understand what new capabilities they need to implement. Implementations can also publish the version number they conform to and that in turn will allow users of OTLP implementations to understand which capabilities of OTLP the implementation supports.

An example of a field that would be labeled is Metric metadata introduced here.

Important: this has nothing to do with having a runtime version number in the proto or elsewhere on the wire (which was a feature that we discussed in the past and decided not to add).

@dashpole @jsuereth and @open-telemetry/specs-approvers