DOCS-3886 Generate Metrics From RUM Events Edits

[alai97]

What does this PR do?

Rebrands RUM-based metrics as custom metrics, adjusts the page headers, and documents the session/view matching criteria dropdown.

Motivation

DOCS-3886

Additional Notes

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Reviewer checklist

• [ ] Review the changed files.
• [ ] Review the URLs listed in the Preview section.
• [ ] Check images for PII
• [ ] Review any mentions of "Contact Datadog support" for internal support documentation.

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/alai97/rum-custom-metrics-doc-update/real_user_monitoring/generate_metrics/