* Alerting: Refactor & fix unified alerting metrics structure
Fixes and refactors the metrics structure we have for the ngalert service. Now, each component has its own metric struct that includes the JUST the metrics it uses. Additionally, I have fixed the configuration metrics and added new metrics to determine if we have discovered and started all the necessary configurations of an instance.
This allows us to alert on `grafana_alerting_discovered_configurations - grafana_alerting_active_configurations != 0` to know whether an alertmanager instance did not start successfully.
* Alerting: Send alerts to external Alertmanager(s)
Within this PR we're adding support for registering or unregistering
sending to a set of external alertmanagers. A few of the things that are
going are:
- Introduce a new table to hold "admin" (either org or global)
configuration we can change at runtime.
- A new periodic check that polls for this configuration and adjusts the
"senders" accordingly.
- Introduces a new concept of "senders" that are responsible for
shipping the alerts to the external Alertmanager(s). In a nutshell,
this is the Prometheus notifier (the one in charge of sending the alert)
mapped to a multi-tenant map.
There are a few code movements here and there but those are minor, I
tried to keep things intact as much as possible so that we could have an
easier diff.
