Add filtered-metrics option

[mlahargou]

This adds a filtered-metrics option. Only metrics included in the multiline
string should be sent out.

I've added some tests for the new function, but let me know if you'd like more exhaustive tests.

Is included-metrics a better name?

Ref: #1081

[mlahargou]

This can be used like:

         - name: Send metrics to Datadog
           uses: int128/datadog-actions-metrics@v1
           with:
              send-pull-request-labels: true
              collect-job-metrics: true
              collect-step-metrics: true
              filtered-metrics: |
                 github.actions.workflow_run.conclusion.failure_total
                 github.actions.workflow_run.duration_second.distribution
                 github.actions.job.conclusion.success_total
                 github.actions.job.conclusion.failure_total
                 github.actions.job.queued_duration_second.distribution
                 github.actions.job.duration_second.distribution
                 github.actions.step.conclusion.failure_total
                 github.actions.step.duration_second.distribution