Fix minor formatting issues in trace metrics documentation

[t5kaji]

What does this PR do? What is the motivation?

This PR fixes minor formatting inconsistencies in the trace metrics documentation, including

• Ensuring the Note block is visually separated from the description
• Displaying Metric Type as a separate block for better clarity
• Renaming XRay to X-Ray to align with the official naming

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/takuya.kajiwara/fix-trace-metrics-formatting/tracing/metrics/metrics_namespace

[t5kaji]

/merge

[dd-devflow]

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2025-03-27 08:46:14 UTC ℹ️ Start processing command /merge

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2025-03-27 08:46:20 UTC ℹ️ MergeQueue: waiting for PR to be ready

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2025-03-27 12:47:05 UTC ⚠️ MergeQueue: This merge request was unqueued

devflow unqueued this merge request: It did not become mergeable within the expected time

[estherk15]

/merge

[dd-devflow]

View all feedbacks in Devflow UI.
2025-03-27 16:49:56 UTC ℹ️ Start processing command /merge

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2025-03-27 16:50:03 UTC ℹ️ MergeQueue: pull request added to the queue

The expected merge time in master is approximately 16m (p90).

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2025-03-27 17:51:27 UTC ℹ️ MergeQueue: This merge request was merged