[prometheusremotewriteexporter] Support sending trace id and span id for exemplars

[dashpole]

Description:
Fixes #8351. This is aligned with the specification for OTLP -> prometheus.

The specification does not say how to handle collisions between attributes named trace_id and span_id and the trace id or span id of the exemplar itself. This PR has attributes take precedence over trace id and span id from the exemplar itself.

Testing:

Unit test checks that trace id and span id are added as labels on prometheus exemplars.

[mx-psi]

Just one nit to make this more consistent with the rest of the codebase

[dashpole]

cc @anneelisabethlelievre, who initially implemented exemplars for PRW
cc @Aneurysm9

[jpkrohling]

@dashpole, could you please address the conflict? Given the approvals, I'll merge this PR afterward.

[dashpole]

It is non-trivial to rebase (since we now need to handle max exemplar size including these new values), so i'd like another review from someone before we merge.

[dashpole]

The additional change is that we have to keep track of the number of runes for the new trace id and span id to make sure the total doesn't exceed limits. If it does exceed the limit, we need to keep trace and span id, and ignore filtered attributes. The relevant line from the spec is: filtered_attributes MUST be added as labels on the OpenMetrics exemplar unless they would exceed the OpenMetrics limit on characters. I've added a unit test which tests that case

[dashpole]

Good to go now!