feat: support metrics of events matched

[seethedoor]

Now we have metrics of events_sent, which means total calculation of events sent from api-server and received by the event-exporter. But we don't have metrics to show how many events matched and sent to receivers we defined.

This PR supports metrics of events-matched metrics. It will create metrics named event_exporter_events_matched, and the matched rule name is given as a tag of the metrics. All the matched events will be calculated to different tags of the event_exporter_events_matched metrics. like this:

• rule example

  route:
  routes:
  - match:
      - receiver: "alerts"
        reason: "FailedScheduling"
        name: "FailedScheduling"
  - match:
      - reason: ScalingReplicaSet
        receiver: alerts
        name: "ScalingReplicaSet"
  - match:
      - reason: Kill
        receiver: alerts

• metrics example

  event_exporter_events_matched{route="FailedScheduling"} 2
  event_exporter_events_matched{route="ScalingReplicaSet"} 12
  event_exporter_events_matched{route="default"} 15

name property of rule can be defined, and if it is empty, the matched events will be calculated to the metrics with default name-tag.

issue: #169