Implement HTTP API for getting all label names

[juliusv]

Would be relevant for grafana/grafana#6038 (cc @bergquist).

Endpoint would be:

GET /api/v1/labels

Output would be:

{
   "status" : "success",
   "data" : [
      "__name__",
      "job",
      "instance",
      "method",
      "status",
      "path"
   ]
}

This isn't a trivial change, as it requires plumbing all the way down to the storage layer and the LevelDB index accessor functions.

[opsnull]

any updating ?

[brancz]

I don't think we'll tackle this before 2.0 as LevelDB is going away completely for that and this feature would have to be rewritten entirely. @fabxc what is your stance on getting something like this into the 2.0 dev branch? However I wouldn't expect this to be too difficult with the 2.0 storage given that we are already keeping indices of labels and their values, right @fabxc ?

[fabxc]

Yea, that should be reasonably easy to do. I also want to investigate composite indices against metric names then, i.e. "what are all possible values of label instance for metric requests_total".

[juliusv]

@fabxc @gouthamve This just came up again in a discussion with @davkal (it could be useful for the UIs he's building). Do you have any more insight into how to do this efficiently on top of the new storage? To do it properly with minimal overhead, it'd probably need a new TSDB interface function?

[gouthamve]

So the tsdb.Querier already exposes it: https://github.com/prometheus/tsdb/blob/master/querier.go#L34-L35

We need to add it to the Storage interface I guess.

[shubheksha]

@gouthamve, that returns all the values, not names. A new function will be needed. See prometheus/tsdb#279

[codesome]

With prometheus/tsdb#369 being merged, I will be adding this API soon!

[krasi-georgiev]

the work on tsdb side is done so removed the local storage label.