feat: Add Grafana dashboard for Prometheus metrics

docs/guides/metrics.mdx

@@ -47,3 +47,7 @@ All metrics are prefixed with `continuwuity_`.
 | `tokio_global_queue_depth` | gauge | | Tasks in the tokio global queue |
 | `tokio_worker_busy_seconds_total` | counter | | Time worker threads spent busy |
 | `tokio_worker_parks_total` | counter | | Number of times worker threads parked |
+
+## Grafana dashboard
+
+Import [grafana-dashboard.json](/guides/grafana-dashboard.json) under **Dashboards → New → Import**, or provision it from a file.