vmalert: support configuration file for notifiers
* vmalert notifiers now can be configured via file
see https://docs.victoriametrics.com/vmalert.html#notifier-configuration-file
* add support of Consul service discovery for notifiers config
see https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1947
* add UI section for currently loaded/discovered notifiers
* deprecate `-rule.configCheckInterval` in favour of `-configCheckInterval`
* add ability to suppress logs for duplicated targets for notifiers discovery
* change behaviour of `vmalert_alerts_send_errors_total` - it now accounts
for failed alerts, not HTTP calls.
* vmalert: remove `vmalert_execution_duration_seconds` metric
The summary for `vmalert_execution_duration_seconds` metric gives no additional
value comparing to `vmalert_iteration_duration_seconds` metric.
* vmalert: update config reload success metric properly
Previously, if there was unsuccessfull attempt to reload config and then
rollback to previous version - the metric remained set to 0.
* vmalert: add Grafana dashboard to overview application metrics
* docker: include vmalert target into list for scraping
* vmalert: extend notifier metrics with addr label
The change adds an `addr` label to metrics for alerts_sent and alerts_send_errors
to identify which exact address is having issues.
The according change was made to vmalert dashboard.
* vmalert: update documentation and docker environment for vmalert's dashboard
Mention Grafana's dashboard in vmalert's README in a new section #Monitoring.
Update docker-compose env to automatically add vmalert's dashboard.
Update docker-compose README with additional info about services.
The change introduces new entity `manager` which replaces
`watchdog`, decouples requestHandler and groups. Manager
supposed to control life cycle of groups, rules and
config reloads.
Groups export an ID method which returns a hash
from filename and group name. ID supposed to be unique
identifier across all loaded groups.
Some tests were added to improve coverage.
Bug with wrong annotation value if $value is used in
templates after metrics being restored fixed.
Notifier interface was extended to accept context.
New set of metrics was introduced for config reload.
