docs: filters: log_to_metrics: general doc updates and cleanup

[eschabell]

Add an "Operational considerations" section covering long-running
pipeline behavior that isn't visible in short-lived tests:

• cardinality and process-lifetime state retention for counter/gauge, with guidance to pre-aggregate high-cardinality fields upstream
• how to monitor the internal emitter via /api/v1/metrics and what a stalled emitter looks like
• emitter_mem_buf_limit pause semantics and mitigation knobs
• counter reset behavior across restarts and how Prometheus consumers should query counters

Fixes #2551

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[eschabell]

Merging as reviewer approved.