test(otlp-transformer): add metrics transfrom benchmark

[pichlermarc]

Which problem is this PR solving?

Prepares for #6570 - we currently don't have a good way to measure performance for OTLP metrics. This PR adds a metrics benchmark with 512 metrics.

The number of 512 metrics is abritrary, but helps land the baseline in a similar ballpark to the logs and trace batch sizes. From personal experience the amount of metrics in a message varies widely, so a one-size-fits-most number of metrics is more difficult to find compared to logs and traces.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.70%. Comparing base (a28f12f) to head (122bff6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6628   +/-   ##
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  Coverage   95.70%   95.70%           
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  Files         375      375           
  Lines       12954    12954           
  Branches     3099     3099           
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  Hits        12398    12398           
  Misses        556      556           

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