Document Prometheus sink

[mavam]

🔍 Problem

• The operator reference has no page for the new to_prometheus sink.
• The linked code PR gives users a Prometheus-compatible path by reshaping metric events in TQL and sending them via Remote Write.

🛠️ Solution

• Add reference documentation for canonical input records, expression mapping, Remote Write v1/v2 selection, metadata fields, and current MVP limits.

💬 Review

• Check that option names and defaults match the code PR.

_{⚙️ Code PR: tenzir/tenzir#6189
🎫 References TNZ-428}

[github-actions]

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_{Verified for e1205fa  ·  Auto-updates on push}