Add metrics processor for excluding based on metric label

[sffc]

I'm running the ops agent on a GCE instance. The instance is running an application that uses a large number of loopback devices, like /dev/loop1, /dev/loop2, etc. I do not want metrics to be stored for these loopback devices because it accounts for a substantial portion of the storage cost of my application's metrics.

Currently, all I can find is to disable the entire agent.googleapis.com/disk/* class of metrics, which is a blunt instrument that throws out metrics I want to track, like disk utilization on the main device /dev/sda2.

I'd like to be able to write a processor like this:

metrics:
  processors:
    metrics_filter:
      type: exclude_by_label
      label: device
      label_pattern: /dev/loop*
      metrics_pattern:
      - agent.googleapis.com/disk/*

Explanation: this is a new metrics filter type named exclude_by_label. It takes a label option, indicating the label to consider as part of the filter, and a label_pattern option, indicating pattern to be matched. If the value of the label matches the pattern, then the metric entry should be excluded. I kept metrics_pattern to indicate the metrics to which to apply the filter.

[github-actions]

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