[WIP] Convert LabelsBuilder baseMap to *sync.Map from map[string]string since we share the map now to reduce allocations

[JordanRushing]

What this PR does / why we need it:
This PR is an experiment.

re:

• reduce cost of pipeline stages by pooling maps we create for labels #11484

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• 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

[github-actions]

Trivy scan found the following vulnerabilities:

• HIGH, Target: docker.io/grafana/loki:main-599eed7 (alpine 3.18.4), Type: alpine openssl: Incorrect cipher key and IV length processing in libcrypto3 v3.1.3-r0. Fixed in v3.1.4-r0
• HIGH, Target: docker.io/grafana/loki:main-599eed7 (alpine 3.18.4), Type: alpine openssl: Incorrect cipher key and IV length processing in libssl3 v3.1.3-r0. Fixed in v3.1.4-r0
  \nTo see more details on these vulnerabilities, and how/where to fix them, please run docker build -t grafana/loki:main-599eed7 -f cmd/loki/Dockerfile .
  trivy i grafana/loki:main-599eed7 on your branch. If these were not introduced by your PR, please considering fixing them in via a subsequent PR. Thanks!