Add dynamic duration variable for the repeat_interval

What did you do?

What did you expect to see?

What did you see instead? Under which circumstances?

Environment

• System information:

  insert output of uname -srm here

• Alertmanager version:

  insert output of alertmanager --version here (repeat for each alertmanager
  version in your cluster, if relevant to the issue)

• Prometheus version:

  insert output of prometheus --version here (repeat for each prometheus
  version in your cluster, if relevant to the issue)

• Alertmanager configuration file:

insert configuration here

• Prometheus configuration file:

insert configuration here (if relevant to the issue)

• Logs:

insert Prometheus and Alertmanager logs relevant to the issue here

For managing the repeat interval of the same alert, there is a configuration repeat_interval in the alertmanager below
repeat_interval : <duration>
The <duration> only supports for the pattern that matches regular expression below.
((([0-9]+)y)?(([0-9]+)w)?(([0-9]+)d)?(([0-9]+)h)?(([0-9]+)m)?(([0-9]+)s)?(([0-9]+)ms)?|0)

In my case, I want to set different repeat_interval for each prometheusrule in my k8s cluster.
Now, I can set "route" to change repeat_interval, but it could make the route configuration so dirty.
Example)
If I want to set 4 different repeat_intervals that depend on the alert severity, then there are 4 additional lines for every route config.
Let assume that there are 4 (severity, repeat_interval) pairs. (warning, 1h) (major, 30m) (critical, 10m) and (emergency, 1m)
Then the route should be...

route:
-  receiver: receiver1
   matchers:
   - severity="warning"
     repeat_interval: 1h
-  receiver: receiver1
   matchers:
   - severity="major"
     repeat_interval: 30m
-  receiver: receiver1
   matchers:
   - severity="critical"
     repeat_interval: 10m
-  receiver: receiver1
   matchers:
   - severity="emergency"
     repeat_interval: 1m

If there are several receivers, then the configuration would be 1000+ lines.

If there could be
repeat_inverval: {{ $labels.repeat }} instead of
repeat_interval: static string,
the configuration would be shorter and people could use repeat_interval much more flexible.

Start by reading the Alertmanager configuration and routing behavior around the repeat_interval field, including how alert labels are available to configuration values. Trace the route examples in the issue and determine the supported scope and validation needed for a dynamic duration. Done means repeat intervals can vary by alert as requested without requiring duplicated route entries, with coverage for the documented examples.