Support metrics.go statistics on Loki labels and series API as well.

[kavirajk]

What this PR does / why we need it:
We use metrics.go (a convention) to record some useful statistics about each of the instance and range queries.

So that we can query something like
{foo="bar"}|= "metrics.go" |logfmt|latency=slow

Where latency comes from statistics recorded in pkg/logql/metric.go.

This PR add support those experience for series and labels API as well.

Examples:

1. Labels API

curl -G -s  "http://127.0.0.1:3100/loki/api/v1/label/foo/values"

Generates following log lines via metrics.go

caller=metrics.go:146 org_id=fake latency=fast query_type=labels length=1h0m0s duration=3.07902ms status=200 label=foo throughput=0B total_bytes=0B

2. Series API

curl -G -s  "http://127.0.0.1:3100/loki/api/v1/series" --data-urlencode 'match[]={app="loki"}'

Generates following log lines via metrics.go

level=info ts=2021-12-01T16:06:59.002407Z caller=metrics.go:189 org_id=fake latency=fast query_type=series length=1h0m0s duration=2.773064ms status=200 match="{app=\"loki\"}" throughput=0B total_bytes=0B

Which issue(s) this PR fixes:
Fixes #4503

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated
• Add an entry in the CHANGELOG.md about the changes.

[stale]

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[kavirajk]

closing in regards to #5971