Using sortSeries from Select to decided to sort the response on DistributorQueryable

[alanprot]

What this PR does:

We are always sorting the response even when its not needed.

We still need to sort the series if we are querying multiples backends (ex: ingesters + store-gateways) as the result needs to be sorted in order to be merged here.

Why we don't need to sort when querying only one backend?

• Each backend should be responsible to not returning duplicated timeseries:
  • When querying ingesters the dedup happens here
  • When querying store gateways this flag is ignored as the series are already sorted on tsdb

Why sorting matters?

This sort can add considerable latency when fetching millions of timeseries as it have to sort all those using those tags.

Prometheus itself does similar thing on the remote_read code:

https://github.com/prometheus/prometheus/blob/d56d0a9d52c8614fdf1c5c652061929ede562f3e/storage/remote/read.go#L174

https://github.com/prometheus/prometheus/blob/d56d0a9d52c8614fdf1c5c652061929ede562f3e/storage/remote/codec.go#L148-L164

Im also removing this extra sort as it seems to not be needed as both ingesters and store-gateways should return the labels already sorted as we already make this assumption in other places like here and here

This change will also benefit GetSeries when update prometheus: See prometheus/prometheus#11311

before/after:

Which issue(s) this PR fixes:
Fixes #

Checklist

• [] Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[alvinlin123]

This comment is a bit misleading because I think what you meant that is "if we need to query multiple backend, then we need to merge results which depends on the result from each querier being sorted. We are not sure if each result is sorted default, so it's safer to force sort to ensure the merge can work properly"