Automatic merge from submit-queue (batch tested with PRs 48812, 48276)

Change fluentd-gcp monitoring to use metrics exposed by SD plugin

Following https://github.com/GoogleCloudPlatform/fluent-plugin-google-cloud/pull/135, make fluentd-gcp expose metrics in Prometheus registry and use them instead of counting records in the pipeline.

/cc @piosz @igorpeshansky

```release-note
Fluentd-gcp DaemonSet exposes different set of metrics.
```