[Serverles] fix metrics multiline logs

[maxday]

What does this PR do?

Debug log lines have been added in this PR. Originally, metrics were output in one single line.
Due to a refactor here, SketchSeriesList was moved to in internal package and .String() has become unavailable.

This PR:

• moves String() out of the internal package
• explicitely calls .String() in the debug log line to prevent future failures
• adds a unit test

Before the fix:

After:

Describe how to test/QA your changes

Unit test

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Use the major_change label if your change either has a major impact on the code base, is impacting multiple teams or is changing important well-established internals of the Agent. This label will be use during QA to make sure each team pay extra attention to the changed behavior. For any customer facing change use a releasenote.
• A release note has been added or the changelog/no-changelog label has been applied.
• Changed code has automated tests for its functionality.
• Adequate QA/testing plan information is provided if the qa/skip-qa label is not applied.
• At least one team/.. label has been applied, indicating the team(s) that should QA this change.
• If applicable, docs team has been notified or an issue has been opened on the documentation repo.
• If applicable, the need-change/operator and need-change/helm labels have been applied.
• If applicable, the config template has been updated.

[remeh]

Can you remove this go.sum update from the pull-request?
I think it may be because you have extra/past work done in your directory already which is not yet merged to main or sticked into your go.sum? Not sure. But this is probably better to not commit it as-is as there is many deps changes in there unrelated to your actual fix.

[maxday]

oopsy that's definitely a mistake from my end, this file was not intended to be committed! thanks!

[remeh]

LGTM!