docs: add missing Prometheus metrics to documentation

[Copilot]

Eight metrics exported by prom_export.py were undocumented in the Prometheus integration guide.

Added Metrics Documentation

Connection Health & Performance:

• meshmon_grpc_connection_errors_total - Error counter by type
• meshmon_grpc_connection_duration_seconds - Connection lifetime histogram
• meshmon_grpc_link_utilization_ratio - Link saturation gauge (0-1)
• meshmon_grpc_queue_depth - Outbound message backlog

Timing & Updates:

• meshmon_heartbeat_latency_seconds - Heartbeat RTT histogram
• meshmon_store_update_size_bytes - Update payload size histogram

System Inventory:

• meshmon_info - Version/build metadata
• meshmon_networks_active - Active network count
• meshmon_monitors_active - Per-network monitor count

Supporting Content

Each metric includes:

• PromQL query examples for common use cases
• Alert rules for connection errors, link saturation, and queue backlog
• Recording rules for pre-aggregated metrics
• Grafana dashboard panel suggestions

Fixes

Corrected existing histogram_quantile() queries to use rate() aggregation over histogram buckets:

# Before (incorrect)
histogram_quantile(0.99, meshmon_heartbeat_latency_seconds)

# After (correct)
histogram_quantile(0.99, rate(meshmon_heartbeat_latency_seconds_bucket[5m]))

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