feature request - templated route matcher expressions

Hi,

I see I can define matcher rules like this

route:
  routes:
    - matchers:
        - alertname =~ ".*foo.*"
      receiver: my_receiver

what I'd want would be the possibility to leverage the alerts' labels, and the available functions, to dynamically construct that regex with something like

route:
  routes:
    - matchers:
        - alertname =~ ".*{{ .CommonLabels.some_label  }}.*"
      receiver: my_receiver

and even

route:
  routes:
    - matchers:
        - alertname =~ ".*{{ .CommonLabels.some_label | reReplaceAll 'foo' 'bar'  }}.*"
      receiver: my_receiver

That would let admins define small but dynamic set of rules, that would let users tweak the behavior of the alerting system simply by defining the labels/values of their alerts

The issue provides YAML examples for templated route matcher expressions using alert labels and functions, but names no files or tests. Start by tracing route matcher parsing and template handling; done means the proposed expressions work with label values and functions, with coverage for the examples shown.