**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)_ |
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