Add Grafana Dashboards for Rialto<->Millau Header Relays (#502)

* Scrape Prom data from Header Relayers

* Add Grafana dashboards for Header Sync

* Remove unused dashboard

* Correctly get CPU usage

* Use correct CPU usage source for Eth2Sub dashboard

* Upper case panel title

* Add dashboard for both sides of header exchange

* Add dashboard which uses templating

* Fix TODOs on main header sync dashboards

* Organize individual header sync dashboards a bit better

* Use Rialto and Millau instead of "Source"

* Remove experimental dashaboards