[telemetry] Grafana Cloud dashboard JSON for Charon metrics

[obchain]

Goal

Ship an importable Grafana dashboard so any operator with a Grafana Cloud free-tier account can visualize Charon's Prometheus metrics in one click.

Scope

• New file: deploy/grafana/charon.json — Grafana dashboard model, importable via UI or /api/dashboards/import
• Panels:
  • Scanner: blocks scanned/sec, head lag, WS reconnects
  • Mempool: pending Venus oracle txs/min, impacted positions flagged
  • Executor: simulations/min, sim success rate, builds/min, submits/min
  • Gas: EIP-1559 base fee, priority fee, tx cost in USD cents
  • Profit: realized profit per tx, cumulative profit, failed liquidations
  • Chain health: RPC latency p50/p95, error rate per endpoint
• Variables: chain, instance
• Prometheus datasource populated by charon-metrics exporter on :9091
• Stable dashboard UID + title across re-imports

Depends on

• [telemetry] Prometheus metrics endpoint (liquidations, profit, scan latency, chain health) #20 — metric names must match what the exporter emits

Acceptance

• Import into Grafana Cloud → all panels render (data or "no data" acceptable pre-run)
• README documents the 3-step import flow

[obchain]

Superseded — Grafana dashboard JSON shipped in feat/26-grafana-dashboard.