* vmalert: add flag to limit the max value for auto-resovle duration for alerts
The new flag `rule.maxResolveDuration` suppose to limit max value for
alert.End param, which is used by notifiers like Alertmanager for alerts auto resolve.
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1586
New flag `-rule.configCheckInterval` defines how often `vmalert` will re-read
config file. If it detects any changes, the config will be reloaded.
This behaviour is turned off by default.
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/512
The change adds no new functionality and aims to move flags definitions
to subpackages that are using them. This should improve readability
of the main function.
app/vmalert: add support for TLS configuration
Add support for TLS optional configuration in a similar fashion to what
is currently supported in other vmutils such as vmagent. TLS
configuration options are distinct for datasource, remoteRead,
remoteWrite as well as notifier.
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
