NIFI-6715: Metrics of removed/renamed components continues to remain in PrometheusReportingTask

[kotarot]

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Description of PR

When using PrometheusReportingTask, metrics of removed/renamed components continues to remain after removing/renaming them completely in the NiFi UI. This is supposed to happen because PrometheusReportingTask adds an incremental difference into the Prometheus' collector. To deal with it, we could clear all collectors before returning the metrics. This PR fixes it.

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[kotarot]

Thanks for your review, Pierre!