.maintain/sentry-node: Add monitoring to docker-compose stack

With this patch a user can run the following fully configured and
monitored setup with a single command:

`docker-compose -f .maintain/sentry-node/docker-compose.yml up`

- 2 validators in two different network namespaces, connected via one
sentry node.

- Polkadot-js/apps to connect to one of the nodes above.

- Prometheus scraping the 3 Substrate nodes.

- Grafana displaying data from Prometheus with community dashboards