**What is this feature?**
This PR implements a new Prometheus historian backend that allows Grafana alerting to write alert state history as Prometheus-compatible `ALERTS` metrics to remote Prometheus-compatible data sources.
The metric includes a few additional labels:
* `grafana_alertstate`: Grafana's full alert state, more granular than Prometheus.
* `grafana_rule_uid`: Grafana's alert rule UID.
Grafana states are included in the `grafana_alertstate` label also mapped to Prometheus-compatible `alertstate` values:
| Grafana alert state | `alertstate` | `grafana_alertstate` |
|---------------------|-----------------------|-----------------------|
| `Alerting` | `firing` | `alerting` |
| `Recovering` | `firing` | `recovering` |
| `Pending` | `pending` | `pending` |
| `Error` | `firing` | `error` |
| `NoData` | `firing` | `nodata` |
| `Normal` | _(no metric emitted)_ | _(no metric emitted)_ |
* Rename RecordStatesAsync to Record
* Rename QueryStates to Query
* Implement fanout writes
* Implement primary queries
* Simplify error joining
* Add test for query path
* Add tests for writes and error propagation
* Allow fanout backend to be configured
* Touch up log messages and config validation
* Consistent documentation for all backend structs
* Parse and normalize backend names more consistently against an enum
* Touch-ups to documentation
* Improve clarity around multi-record blocking
* Keep primary and secondaries more distinct
* Rename fanout backend to multiple backend
* Simplify config keys for multi backend mode
Alexander Akhmetov
Alexander Weaver