* vmalert: support rules backfilling (aka `replay`)
vmalert can `replay` configured rules in the past
and backfill results via remote write protocol.
It supports MetricsQL/PromQL storage as data source,
and can backfill data to remote write compatible
storage.
Supports recording and alerting rules `replay`. See more
details in README.
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/836
* vmalert: review fixes
* vmalert: readme fixes
The commit adds a support for template function `query`,
`first` and `value`. The function `query` executes
a MetricsQL query for active alerts. In vmalert we
update templates on every evaluation for active alerts
to keep them up to date. With `query` func it may become
a perf issue since it will fire a query on every execution.
We should keep it in mind for now.
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/539
app/vmalert: Support custom URL for alerts source
Add flag `external.alert.source` for configuring custom URL
for alert's source. This may be handy to re-point default source
URL to other systems like Grafana.
Updates #517
