Adds the ability the fetch label values from the store

[cyriltovena]

Signed-off-by: Cyril Tovena cyril.tovena@gmail.com

This is required to advance grafana/loki#453 in Loki.

I also need to add LabelNames(metricName, start, end) but since this is my first PR I wanted to have a quick feedback on this first.

[bboreham]

Thanks for the PR; it looks promising. This isn't the same as the Prometheus API LabelValues, right? Because that one doesn't have a metric name. I think you should mention that in a comment, and possibly use a different name.

I would probably also split the change into two commits containing the refactor of validateQueryTimeRange() and then the new code.

[cyriltovena]

Not exactly like https://prometheus.io/docs/prometheus/latest/querying/api/#querying-label-values as it requires the metric name like you said.

I'll see how I can split the commit. I actually intentionally squash rebased it to a single commit, since I was used to it for other project.

[cyriltovena]

Could not find a better name but I did add a documentation.

PTAL

[bboreham]

Given that we have LabelValuesForLabelName() elsewhere, how about LabelValuesForMetricName() here?

[bboreham]

Besides the name, LGTM

[tomwilkie]

Nice work Cyril! LGTM too.