Improve performance of labels comparison when querying store-gateways

[charleskorn]

What this PR does

This PR applies #5836 to two additional places that could benefit from it.

This improves performance when using stringlabels (as Mimir does). Note that the existing benchmark includes iterating through all of the samples in the series, which is why there isn't a dramatic latency change in the results below:

goos: darwin
goarch: arm64
pkg: github.com/grafana/mimir/pkg/querier
                                    │ before.txt  │             after.txt             │
                                    │   sec/op    │   sec/op     vs base              │
_blockQuerierSeriesSet_iteration-10   812.2m ± 0%   814.8m ± 0%  +0.32% (p=0.009 n=6)

                                    │  before.txt  │             after.txt              │
                                    │     B/op     │     B/op      vs base              │
_blockQuerierSeriesSet_iteration-10   8.426Mi ± 0%   7.694Mi ± 0%  -8.69% (p=0.002 n=6)

                                    │ before.txt  │             after.txt             │
                                    │  allocs/op  │  allocs/op   vs base              │
_blockQuerierSeriesSet_iteration-10   256.0k ± 0%   240.0k ± 0%  -6.25% (p=0.002 n=6)

Which issue(s) this PR fixes or relates to

(none)

Checklist

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

[pracucci]

Brilliant, thanks!