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

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[clayton-cornell]

Some doc input/comments

[clayton-cornell]

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

[clayton-cornell]

Approved from a docs standpoint

[mattdurham]

Why do we need a separate enabled value? Would having a value inherently make it true?

[hainenber]

Would an empty string indicate of no default instance label for every podlogs or having an empty default instance label?

I'm not sure if having a separate value is the way to go here but I feel like its explicitness would help.

[tpaschalis]

AFAIK in both Prometheus and Loki, an empty value label is considered equivalent to a label that doesn't exist.

So in this case, you wouldn't really be able to have an empty instance label. I'd agree this makes sense as an attribute instead of a block. It should only override defaultInstance when it's expliticly supplied as a non-empty string.

[hainenber]

Very cool knowing the practice put in place by other parts of the ecosystem :D

I'd try implementing it that way.

[hainenber]

I just realized that the original requester would like to disable the behavior of setting default instance label too 🤦

And having an empty string to disable the behavior would be breaking change, unless we choose a token to signify the act of disabling, like defaultInstanceLabel="disabled" or something, which doesn't seem right to me 🤔

To have both the ability to disable the behavior and to set a custom default instance label, I think we gotta resort to block now (unless having a separate attribute :D)

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