Add filter support to prometheus for /api/v1/labels /api/v1/{label}/values

[Hedius]

Not sure if my implementation is really ideal. Atm the querier.go of prometheus does not supprt filter support for metric and label searches. please review if what I do is really ok :)

This pull request implements the following changes:

1. Ensure that the matchers are matched against the Tags Array/Column.
2. For LabelNames() the matchers are performed against the Tags array column (by using index.TaggedTermWhereN)
3. For LabelValues() the same is done + LIKE name=% is added as a where condition for Tag1.

If no matchers are provided graphite-clickhouse should work exactly like before.

If a matcher is provided it is matched against the Tags column.

In our case we use the stack with multi tenant support. So a proxy before graphite-clickhouse performs authentication and injects a label into all requests. {x=y}.

This change ensures that LabelNames() and LabelValues() only returns results which match {x=y}

[msaf1980]

I can't have meny expririece with Prometheus, but looks good.
Branch confilct against current muster need be resolved.
And no integrations tests for prometheus (and other API at now, obly for graphite). It's bad, but I haven't free time for that.
Do you have free time and desire for add prometheus integration tests ?

[Hedius]

Hi, will fix the conflict + will look into writing tests when I have time. maybe this or next week.

[msaf1980]

Hi, will fix the conflict + will look into writing tests when I have time. maybe this or next week.

Hi, it's good news. Also merge with current muster for work test (one of clickhouse version is broken and integrations tests failed).

[Hedius]

So, but now. Will look into this in the coming 2 weeks.

Also will merge master into it again.

[Hedius]

Will have to check how the feature flags added in #266 interact with prometheus.

[Hedius]

Updated to match master again. Works as described above.

I am a little lost at the moment how writing test for prometheus can or should be done, since there are not any at the moment. @bzed Maybe we both can look into this?

Otherwise, at the moment I would avoid doing it.