[NIFI-15162] Flow metrics reporting strategy for nifi metrics api

[esecules]

Summary

NIFI-15162
This reintroduces the metrics reporting strategy feature from the PrometheusReportingTask. It can be used to only collect flow metrics related to process groups. This will help reduce the CPU load of collection metrics from larger flows when the flow admin wants to omit individual processor and connection metrics already.

Tracking

Please complete the following tracking steps prior to pull request creation.

Issue Tracking

• Apache NiFi Jira issue created

Pull Request Tracking

• Pull Request title starts with Apache NiFi Jira issue number, such as NIFI-00000
• Pull Request commit message starts with Apache NiFi Jira issue number, as such NIFI-00000

Pull Request Formatting

• Pull Request based on current revision of the main branch
• Pull Request refers to a feature branch with one commit containing changes

Verification

Please indicate the verification steps performed prior to pull request creation.

Build

• Build completed using ./mvnw clean install -P contrib-check
  • JDK 21
  • JDK 25

Licensing

• na New dependencies are compatible with the Apache License 2.0 according to the License Policy
• na New dependencies are documented in applicable LICENSE and NOTICE files

Documentation

• Documentation formatting appears as expected in rendered files