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