Adding Inital Metrics

[goobaby]

What does this PR do? What is the motivation?

Addes metrics explainmation similar to APM Metrics Pages

Merge instructions

Merge readiness:

• Ready for merge

For Datadog employees:

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/amit.deb/llm_metrics/llm_observability/monitoring/metrics

[cswatt]

Added DOCS-12235 to track review

[cswatt]

I pushed some minor syntax/formatting changes, and have two questions/suggestions for metric descriptions.