Document metrics temporality for time series data streams

[JonasKunz]

Summary

Adds documentation for the metrics temporality feature (GA in 9.5) in time series data streams (TSDS):

• New page: Metrics temporality — Explains the difference between cumulative and delta temporality for counter and histogram metrics, how to configure temporality via the index.time_series.temporality_field index setting, and how temporality affects ES|QL time series queries (rate, increase, irate) and downsampling.
• Updated TSDS overview — Revised counter and histogram metric type descriptions to reference temporality and link to the new page. Added a "Learn more" link for metrics temporality.
• Updated downsampling concepts — Documents temporality-aware downsampling behavior for counters (delta counters store the sum) and histograms (cumulative histograms use the same algorithm as cumulative counters).
• Updated OTLP endpoint docs — New section explaining that the OTLP endpoint automatically preserves metric temporality in a temporality dimension, with a note on histogram field type requirements.
• Navigation — Added the new page to the TSDS advanced topics TOC.

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🔍 Preview links for changed docs

• manage-data/data-store/data-streams/advanced-topics-tsds.md
• manage-data/data-store/data-streams/downsampling-concepts.md
• manage-data/data-store/data-streams/metric-temporality.md
• manage-data/data-store/data-streams/time-series-data-stream-tsds.md
• manage-data/ingest/otlp-endpoint.md

[github-actions]

Elastic Docs Style Checker (Vale)

Summary: 6 warnings found

⚠️ Warnings (6): Fix when the suggestion improves clarity or correctness.

File	Line	Rule	Message
manage-data/data-store/data-streams/downsampling-concepts.md	78	Elastic.Latinisms	Latin terms and abbreviations are a common source of confusion. Use 'versus' instead of 'vs'.
manage-data/data-store/data-streams/downsampling-concepts.md	79	Elastic.Latinisms	Latin terms and abbreviations are a common source of confusion. Use 'versus' instead of 'vs'.
manage-data/data-store/data-streams/downsampling-concepts.md	86	Elastic.Latinisms	Latin terms and abbreviations are a common source of confusion. Use 'versus' instead of 'vs'.
manage-data/data-store/data-streams/metric-temporality.md	46	Elastic.DontUse	Don't use 'just'.
manage-data/data-store/data-streams/metric-temporality.md	72	Elastic.Spelling	'temporalities' is a possible misspelling.
manage-data/data-store/data-streams/metric-temporality.md	101	Elastic.EndPuntuaction	Don't end headings with punctuation.

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[kkrik-es]

Liam may want to take a quick look too.