.maintain/sentry-node: Add monitoring to docker-compose stack (#5321)

* Substrate Dashboard example

* Improve README

* Update README_dashboard.md

* Add screenshots

* Minor fix

* Minor fix, image link

* .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

* .maintain/monitoring/grafana: Change default datasource name

* .maintain/monitoring/grafana: Add metric namespace option

* .maintain/monitoring/grafana: Remove `host` metric from most metrics

* .maintain/monitoring/grafana: Remove underscore from metric_namespace

* .maintain/monitoring: Use `instance` label instead of `hostname`

To identify a scrape target, one should use `instance` and not
`hostname` as multiple targets might run on the same node.

See https://prometheus.io/docs/concepts/jobs_instances/

 for details.

* .maintain/monitoring: Introduce instance variable

* .maintain/monitoring/grafana: Rename substrate_block_height_number

* .maitain/monitoring/grafana: Use instance instead of host in legend

* .maintain/monitoring: Remove node exporter dependency

* .maintain/sentry-node/prometheus: Simplify configuration

* .maintain/monitoring/grafana: Update README and remove images

* .maintain/sentry-node: Improve docs

* .maintain/monitoring/grafana: Use metric_namespace template variable

* Use --sentry from v0.7.29 instead of a reserved-node

* .maintain/sentry-node: Revert sentry-a using validator-b as bootnode

Co-authored-by: DerFredy - @derfredy:matrix.org <[email protected]>
Co-authored-by: david <[email protected]>