feat(loki/src/k8s): configurable default `instance` label for scraped pod logs

[hainenber]

PR Description

Which issue(s) this PR fixes

Fixes grafana/alloy#211

Notes to the Reviewer

PR Checklist

• [x] CHANGELOG.md updated
• [x] Documentation added
• [x] Tests updated
• ~~Config converters updated~~

[clayton-cornell]

Over to @grafana/grafana-agent-maintainers for a code review.

[github-actions[bot]]

This PR has not had any activity in the past 30 days, so the needs-attention label has been added to it. If you do not have enough time to follow up on this PR or you think it's no longer relevant, consider closing it. The needs-attention label signals to maintainers that something has fallen through the cracks. No action is needed by you; your PR will be kept open and you do not have to respond to this comment. The label will be removed the next time this job runs if there is new activity. Thank you for your contributions!

[github-actions[bot]]

This PR has not had any activity in the past 30 days, so the needs-attention label has been added to it. If you do not have enough time to follow up on this PR or you think it's no longer relevant, consider closing it. The needs-attention label signals to maintainers that something has fallen through the cracks. No action is needed by you; your PR will be kept open and you do not have to respond to this comment. The label will be removed the next time this job runs if there is new activity. Thank you for your contributions!

[thampiotr]

Closing as the linked issue is closed too.