Add statistics to query_range and instant_query API.

[cyriltovena]

This introduces a new statistics property to the query_range, instant_query API and also the old query API.

The frontend also honor and merge those statistics although there is two small caveats:

• it doesn't account for time spent in the frontend (planning to improve this in another PR), only queriers exec time is computed.
• result_cache discard statistics (I don't think we want to account for the result_cache anyways)

I think it would be useful may be to add more frontend (eg. amount of split, result cache hit) and chunk/index cache statistics in the long term. But this seems like a good start.

The statistics property contains informations about the execution of the query such as processed lines/bytes per seconds from storage and ingesters.

This is a draft PR as I have to rebase and add API documentation for this.

PS: Unfortunately the Cortex result cache used for metrics requires protof response type and since Cortex wasn't using the same gogoproto version I had to downgrade our version to (1.2.1) until we are in sync, there is only minor bugfix that we don't use in 1.3.0 and I've also opened a PR in Cortex to make the change. See cortexproject/cortex#2055

[rfratto]

This is looking great! I just have one question about how it's exposed to the user. I remember some internal conversations about matching what Prometheus does.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3ab590b). Click here to learn what that means.
The diff coverage is 86.25%.

@@            Coverage Diff            @@
##             master    #1615   +/-   ##
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  Coverage          ?   61.69%           
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  Files             ?      109           
  Lines             ?     8260           
  Branches          ?        0           
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  Hits              ?     5096           
  Misses            ?     2770           
  Partials          ?      394

Impacted Files	Coverage Δ
pkg/iter/iterator.go	60.48% <0%> (ø)
pkg/loki/modules.go	11.29% <0%> (ø)
pkg/loghttp/query.go	47.36% <0%> (ø)
pkg/querier/http.go	7.06% <0%> (ø)
pkg/logql/marshal/marshal.go	75.86% <100%> (ø)
pkg/querier/queryrange/prometheus.go	89.18% <100%> (ø)
pkg/logql/marshal/legacy/marshal.go	63.63% <100%> (ø)
pkg/querier/queryrange/codec.go	95.68% <100%> (ø)
pkg/logql/stats/context.go	92.23% <100%> (ø)
pkg/querier/querier.go	70.06% <100%> (ø)
... and 7 more

[cyriltovena]

This is ready for reviews.

/cc @owen-d

[rfratto]

LGTM other than two indentation problems

[rfratto]

LGTM 🎉