Document metrics that help detect when WAL apply lag is increasing

[mtopolnik]

Adds three new metrics to the table of all metrics:

1. counter: questdb_wal_apply_seq_txn_total
2. counter: questdb_wal_apply_writer_txn_total
3. gauge: questdb_suspended_tables

TODO: document how to use these metrics:

To track increasing WAL apply lag not caused by suspended tables, monitor the difference questdb_wal_apply_seq_txn_total - questdb_wal_apply_writer_txn_total. If it's persistently growing, it is likely that at least one table is falling seriously behind.

The gauge questdb_suspended_tables is simpler, any value above zero is a sign of trouble.

[github-actions]

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[Copilot]

Pull Request Overview

This PR introduces new observability metrics for WAL apply lag and a dedicated monitoring guide, while updating documentation structure.

• Adds three new Prometheus metrics for WAL apply and suspended tables
• Introduces a new “Monitoring and alerting” guide and links it in the sidebar
• Refactors existing logging-metrics wording for clarity

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
documentation/third-party-tools/prometheus.md	Added questdb_wal_apply_seq_txn_total, questdb_wal_apply_writer_txn_total, questdb_suspended_tables and updated table separator
documentation/sidebars.js	Inserted "operations/monitoring-alerting" into the docs sidebar
documentation/operations/monitoring-alerting.md	Created a new guide covering suspended tables alerts and WAL apply lag detection
documentation/operations/logging-metrics.md	Refactored the CPU-cores health check paragraph and adjusted surrounding text

Comments suppressed due to low confidence (1)

documentation/operations/logging-metrics.md:213

• [nitpick] This sentence fragment lacks its introductory context (e.g., 'On systems with 8 cores or less...'). Please reintroduce or rephrase the initial clause for clarity.

for threads might increase the latency of health check service responses. If you