Add a prometheus label mapping component

[vaxvms]

PR Description

This PR add a prometheus component to create a label based on a source_label value and a static mapping table.

Which issue(s) this PR fixes

For a large mapping table, using regex with prometheus.relabel is inefficient

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated

[CLAassistant]

All committers have signed the CLA.

[clayton-cornell]

Some initial doc review comments

[thampiotr]

Thanks for contributing.

For a large mapping table, using regex with prometheus.relabel is inefficient

Could you share the use case and performance differences? In many cases users can use discovery.relabel to add the team=X labels - and then the overhead is much smaller, because it's not samples, but targets that are relabeled.

If the reason to add this is that prometheus.relabel it's not efficient, we will need some more data on how inefficient it is and whether it could be optimised to work faster. Any new solution we would consider will require benchmarks to prove that it's more efficient. We also generally want to avoid having many ways of doing the same thing - but there can be exceptions.

[vaxvms]

I have to map a label holding the service ID to a label holding the tenant ID.

My test case is using X services ID mapped to Y tenants. My test case is mapping 100 different services ID to a single tenant.
Obviously, Y<=X.

Using prometheus.relabel, with X set to 20k (and Y to 200), i'm consuming 20 vCPU. I'm also having a 49% failure rate

Using prometheus.mapping, with X set to 100k (and Y to 1k), i'm consuming 2 vCPU

As my worst case can be X == Y, i haven't try to summarize regex so i have a rule block for each service ID