[DOCS-14454] Add description front matter to top-level metrics pages

[buraizu]

What does this PR do? What is the motivation?

Fixes DOCS-14454

Adds the description field to YAML front matter on 7 top-level metrics pages that lacked it. Per the Datadog docs front matter guidance, description is required on every page.

This PR covers top-level metrics pages only. Follow-up PRs will cover metrics/guide/, metrics/custom_metrics/, metrics/faq/, and metrics/open_telemetry/.

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/docs14454/metrics-toplevel-descriptions/metrics/
• https://docs-staging.datadoghq.com/docs14454/metrics-toplevel-descriptions/metrics/derived-metrics
• https://docs-staging.datadoghq.com/docs14454/metrics-toplevel-descriptions/metrics/metrics-without-limits
• https://docs-staging.datadoghq.com/docs14454/metrics-toplevel-descriptions/metrics/nested_queries
• https://docs-staging.datadoghq.com/docs14454/metrics-toplevel-descriptions/metrics/overview
• https://docs-staging.datadoghq.com/docs14454/metrics-toplevel-descriptions/metrics/types
• https://docs-staging.datadoghq.com/docs14454/metrics-toplevel-descriptions/metrics/units

[datadog-prod-us1-4]

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 1 Pipeline job failed

DataDog/documentation | missing_tms_preview     

🛟 This job is unlikely to succeed on retry. Please review your pipeline configuration.

Missing trademarks in multiple HTML files and subsequent Slack notification failed due to missing AWS parameter.

Useful? React with 👍 / 👎

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 832a94a | Docs | Datadog PR Page | Give us feedback!}