Add note for metrics yaml definitions

[trask]

This isn't a blocker, as we can add the note manually to the markdown file for now, but it would be nice to add the note in the yaml definition.

Related to open-telemetry/opentelemetry-specification#3418 and open-telemetry/opentelemetry-specification#3352 (comment)

[Oberon00]

I think this is open-telemetry/semantic-conventions#1086. The note/brief for the group is currently missing everywhere in the markdown output, except for the attributes themselves. If you agree, I'd close this as a duplicate.

[joaopgrassi]

I took a quick look, I think it should be fairly easy to add/render the notes for metrics. Metrics render the brief on a description field in the table and in there we can just append the note [x] as we do for attributes.

But for other things like resource conventions which we don't have any special rendering in markdown we might need to spend more time thinking/working on it.

So, if you think it's OK, we can leave this issue open and work on adding note support for metrics only and work on the rest as part of open-telemetry/semantic-conventions#1086.

I can take and work on those individually.