chore: Improve performance of structured metadata

[cyriltovena]

What this PR does / why we need it:

I realized queries with structured metadata where a bit slow and turns out we do a lot allocations in fact a slice of labels for each log line.

Improving this pretty impressive result:

❯ benchstat before.txt after.txt
name                        old time/op    new time/op    delta
BackwardIterator/66_kB-16     26.6ms ±11%    20.7ms ±11%  -22.12%  (p=0.008 n=5+5)
BackwardIterator/262_kB-16    21.8ms ±14%    19.3ms ± 8%  -11.75%  (p=0.016 n=5+5)
BackwardIterator/524_kB-16    16.7ms ± 6%    18.5ms ±19%     ~     (p=0.421 n=5+5)

name                        old alloc/op   new alloc/op   delta
BackwardIterator/66_kB-16     8.91MB ± 0%    1.02MB ± 0%  -88.53%  (p=0.008 n=5+5)
BackwardIterator/262_kB-16    7.34MB ± 0%    0.33MB ± 2%  -95.49%  (p=0.008 n=5+5)
BackwardIterator/524_kB-16    5.88MB ± 0%    0.13MB ± 3%  -97.77%  (p=0.008 n=5+5)

name                        old allocs/op  new allocs/op  delta
BackwardIterator/66_kB-16      51.5k ± 0%      2.8k ± 0%  -94.64%  (p=0.000 n=5+4)
BackwardIterator/262_kB-16     44.0k ± 0%      0.6k ± 0%  -98.60%  (p=0.008 n=5+5)
BackwardIterator/524_kB-16     36.0k ± 0%      0.3k ± 0%  -99.29%  (p=0.008 n=5+5)

cc @sandeepsukhani do you see anything wrong ?

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[cyriltovena]

I wonder if this is fine to reuse the same buffer. cc @sandeepsukhani

[sandeepsukhani]

LGTM