Support series selector for label/names and label/:name/values API

[jobrs]

Proposal

Add support for series selectors (i.e. match[]) to the label/names resp. label/:name/values APIs, so that the list of label names and values can be constrained to a subset of series.

This is useful to build

• Multi-tenancy using on labels for separating tenants (similar to #1813 (comment)_): To constrain a API call to a tenant you need to add a filter like tenant=xyz.
• Sophisticated UIs supporting multiple filter conditions: To narrow down possible values iteratively (see also #407)

You could use the /series API for that purpose but that's quite a complex operation (example). I am hoping that this could be implemented in a much more efficient way on a lower layer.

[codesome]

This discussion had come up during the implementation of /label/names, but there was no efficient way to do it as of now. The best way now is equivalent to querying the tsdb for series and filter the required information - which is inefficient.

[jobrs]

@codesome Agreed, see my last sentence. Still I would find the API addition useful, since future versions of the TSDB might (wishful thinking) allow for an optimised implementation and the addition would be consistent with the rest of the API.

[codesome]

Quoting prometheus/tsdb#530 (comment) from @brian-brazil

That's the only way to do it and the most efficient, I don't see the need to extend the API.

[krasi-georgiev]

@jobrs is there anything else we need to add the the issue?
Otherwise I agree that we should extend the API only when we have a more efficient way that the current implementation.

[jobrs]

@krasi-georgiev no, these are the points I wanted to raise

[krasi-georgiev]

In that case I hope you don't mind closing the issue and can revisit at a later point when we can have a better answer.

[jobrs]

closing this one, since the requested API addition is not needed from functional and performance perspective