add test for OTLP metrics partial success response

[endorama]

Motivation/summary

We recently introduced OTLP partial success response support in #11883. This PR adds a test case for OTLP metrics to ensure we are responding using a PartialSuccess.

Note that this test rely on triggering a partial response from the server by sending malformed Exponential Histogram data.

Checklist

• Update CHANGELOG.asciidoc
• Update package changelog.yml (only if changes to apmpackage have been made)
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Run systemtests, verify that TestOTLPGRPCMetrics_partialSuccess passes.

Related issues

Ref: #12226

[mergify]

This pull request does not have a backport label. Could you fix it @endorama? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.

[carsonip]

thanks!