Add all Prometheus metrics to documentation

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Summary

• Added all Lightdash-specific Prometheus metrics to the documentation page, sourced from the codebase (packages/backend/src/prometheus/PrometheusMetrics.ts)
• New metric sections added: Query metrics (7), Pre-aggregate metrics (16), AI agent metrics (4), S3 metrics (1), and Custom event metrics
• Added LIGHTDASH_CUSTOM_METRICS_CONFIG_PATH environment variable to the configuration table
• Existing sections (Process, Node.js, PostgreSQL, Queue) remain unchanged
• Added labels column to PostgreSQL metrics table for consistency with new sections

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Project	Status	Preview	Updated (UTC)
lightdash	🟢 Ready	View Preview	Apr 15, 2026, 8:22 AM

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