chore(monitoring): add Prometheus/Grafana docker-compose stack

Scrapes electrs metrics from a configurable target (default
10.9.68.11:4224). Target is set via monitoring/.env and substituted
into the Prometheus config at container startup with sed.

Includes a RocksDB dashboard covering: write stalls, L0 file count,
files per level, pending compaction, memtable pressure, block cache
usage, SST file sizes, key count, and background errors.

Author: philippem

monitoring/.env

@@ -0,0 +1 @@
+ELECTRS_TARGET=10.9.68.11:4224

monitoring/docker-compose.yml

@@ -0,0 +1,40 @@
+services:
+  prometheus:
+    image: prom/prometheus:latest
+    ports:
+      - "9090:9090"
+    volumes:
+      - ./prometheus:/etc/prometheus:ro
+      - prometheus_data:/prometheus
+    environment:
+      - ELECTRS_TARGET=${ELECTRS_TARGET}
+    entrypoint:
+      - /bin/sh
+      - -c
+      - |
+        sed "s|__ELECTRS_TARGET__|$$ELECTRS_TARGET|g" /etc/prometheus/prometheus.yml > /tmp/prometheus.yml
+        exec /bin/prometheus \
+          --config.file=/tmp/prometheus.yml \
+          --storage.tsdb.path=/prometheus \
+          --storage.tsdb.retention.time=30d
+    restart: unless-stopped
+
+  grafana:
+    image: grafana/grafana:latest
+    ports:
+      - "3000:3000"
+    volumes:
+      - ./grafana/provisioning:/etc/grafana/provisioning:ro
+      - ./grafana/dashboards:/var/lib/grafana/dashboards:ro
+      - grafana_data:/var/lib/grafana
+    environment:
+      - GF_SECURITY_ADMIN_PASSWORD=admin
+      - GF_AUTH_ANONYMOUS_ENABLED=true
+      - GF_AUTH_ANONYMOUS_ORG_ROLE=Viewer
+    depends_on:
+      - prometheus
+    restart: unless-stopped
+
+volumes:
+  prometheus_data:
+  grafana_data: