[out_loki] Add support for structured metadata

[thomaschaaf]

Is your feature request related to a problem? Please describe.
Searching for requestIds is slow with loki. We would like to provide it with structured metadata so that it is faster at searching for things like request ids.

Describe the solution you'd like
Allow to send structured metadata: https://grafana.com/docs/loki/latest/get-started/labels/structured-metadata/

[ptr1120]

+1

[github-actions]

This issue is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 5 days. Maintainers can add the exempt-stale label.

[jtackaberry]

Not stale. Necessary feature as many of us are using labels to encode information that really should be structured metadata (such as certain annotations coming from K8s pods).

[jtackaberry]

Loki 3.0 is now out and structured metadata is officially stable.

[jtackaberry]

I've submitted PR #9048 for this one.