docs: fixed typos/grammatical mistakes in metrics.md

[ceosion]

What this PR does / why we need it:

Fixes a few typos & grammatical mistakes in metrics.md

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

N/A

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• [n/a] 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.
• [n/a] Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• [n/a] 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

[JStickler]

[docs team] LGTM, thanks for taking the time to update the docs.