Opmize GetLabels and GetLabels values from store gateway

[alanprot]

What this PR does:
Right now, we wait the result from all store gateways be returned to only then Merge/Dedup the response.

This change is similar to what was done on: #5785

➜  ~ ~/go/bin/benchstat /tmp/old /tmp/new
goos: darwin
goarch: arm64
pkg: github.com/cortexproject/cortex/pkg/querier
                             │  /tmp/old   │              /tmp/new              │
                             │   sec/op    │   sec/op     vs base               │
BlocksStoreQuerier_Labels-10   3.013m ± 1%   2.147m ± 0%  -28.76% (p=0.002 n=6)

                             │   /tmp/old   │              /tmp/new               │
                             │     B/op     │     B/op      vs base               │
BlocksStoreQuerier_Labels-10   8.832Mi ± 0%   2.151Mi ± 0%  -75.64% (p=0.002 n=6)

                             │  /tmp/old   │             /tmp/new              │
                             │  allocs/op  │  allocs/op   vs base              │
BlocksStoreQuerier_Labels-10   20.06k ± 0%   19.84k ± 0%  -1.10% (p=0.002 n=6)

Which issue(s) this PR fixes:
Fixes #

Checklist

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

[yeya24]

@alanprot Can we provide some kind of go benchmark results?

[alanprot]

@yeya24 Update the PR to use the MergeSlicesParallel and added a bench.

It seems better but maybe we need more tests? like with different number of labels being returned?