Poc cost attribution

[ying-jeanne]

What this PR does

The prototype to support adding custom label to track active series/ samples received / discarded samples related to this ticket https://github.com/grafana/mimir/issues/5698.

The cost attribution related metrics would be exported through a different endpoint.

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• [ ] Tests updated.
• [ ] Documentation added.
• [ ] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [ ] about-versioning.md updated with experimental features.

[bboreham]

Glad to see this make some progress. Ideally I would like to see this kind of implementation take over from usage groups, in simple cases. So it would be good to have the label names, etc., line up.