Fix: make Grafana optional and allow README in build

monitoring/README.md

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+# Monitoring integration
+
+GuardDen exposes Prometheus metrics from the bot and dashboard services. You can
+keep using your existing Grafana instance by pointing it at your Prometheus
+server (yours or the optional one in this repo).
+
+## Option A: Use the bundled Prometheus, external Grafana
+
+Start only Prometheus:
+
+```bash
+docker compose --profile monitoring up -d prometheus
+```
+
+Add a Prometheus datasource in your Grafana:
+- URL from the same Docker network: `http://guardden-prometheus:9090`
+- URL from the host: `http://localhost:9090` (or your mapped port)
+
+## Option B: Use your own Prometheus + Grafana
+
+Merge the scrape jobs from `monitoring/prometheus.yml` into your Prometheus
+config. The main endpoints are:
+- bot: `http://<bot-host>:8001/metrics`
+- dashboard: `http://<dashboard-host>:8000/metrics`
+- postgres-exporter: `http://<pg-exporter-host>:9187/metrics`
+- redis-exporter: `http://<redis-exporter-host>:9121/metrics`
+
+Then add (or reuse) a Prometheus datasource in Grafana that points to your
+Prometheus URL.
+
+## Optional internal Grafana
+
+If you want the repo's Grafana container, enable its profile:
+
+```bash
+docker compose --profile monitoring --profile monitoring-grafana up -d
+```