Add visible_to with the same support for templating and fields as the
existing responders field.

Opsgenie documentation:

  Teams and users that the alert will become visible to without sending
  any notification. type field is mandatory for each item, where possible
  values are team and user. In addition to the type field, either id or
  name should be given for teams and either id or username should be given
  for users. Please note: that alert will be visible to the teams that are
  specified within responders field by default, so there is no need to
  re-specify them within visibleTo field. You can refer below for example
  values.

The functionality has been tested with unit tests, and with the following
configuration:

  route:
    receiver: og
    group_wait: 30s
    group_interval: 5m
    repeat_interval: 12h
  receivers:
  - name: og
    opsgenie_configs:
    - send_resolved: true
      api_key: ...top secret...
      api_url: https://api.eu.opsgenie.com/
      message: 'Lab message'
      description: 'Static description'
      source: lab
      details:
        foo: bar
      priority: P4
      visible_to:
      - type: user
        username: [email protected]

Fixes prometheus#3953

Signed-off-by: Carl Henrik Lunde <[email protected]>

Signed-off-by: Carl Henrik Lunde <[email protected]>