NIFI-16298 Expose Primary Node and Cluster Coordinator role as Prometheus metrics

[abij]

Summary

NIFI-16298 - Expose Primary Node and Cluster Coordinator role as Prometheus metrics

The Prometheus flow metrics endpoint (/nifi-api/flow/metrics/prometheus) exposes cluster membership/connectivity metrics (cluster_is_clustered, cluster_is_connected_to_cluster, cluster_connected_node_count, cluster_total_node_count), but nothing indicates which node currently holds the Primary Node or Cluster Coordinator role. Building a per-node cluster-overview dashboard currently has no way to mark those roles without querying NiFi's REST API separately and joining the data out-of-band.

The data is already available internally and simply wasn't wired into these metrics: FlowController.isPrimary() and FlowController.isClusterCoordinator() already exist, following the exact same pattern as FlowController.isClustered(), which ControllerFacade/StandardNiFiServiceFacade already use for the existing cluster metrics.

What this PR does

• Adds ControllerFacade#isPrimary() and ControllerFacade#isClusterCoordinator(), delegating to the equivalent FlowController methods — following the exact pattern of the existing ControllerFacade#isClustered() passthrough.
• Adds two new gauges to ClusterMetricsRegistry, labeled instance only (consistent with the other single-value cluster gauges): cluster_is_primary_node and cluster_is_cluster_coordinator.
• Wires them into PrometheusMetricsUtil#createClusterMetrics and StandardNiFiServiceFacade#populateFlowMetrics alongside the existing cluster metrics.
• Updates TestFlowResource's fixture and sample-count assertions for the two new metrics.

This lets a dashboard mark the Primary Node and Cluster Coordinator directly from the same Prometheus scrape already used for the rest of the cluster-overview table, without a separate REST API call/join. Also useful for alerting (e.g. "no node in this cluster is reporting as coordinator" indicates a leader-election problem).

Tests

• ./mvnw -pl nifi-framework-bundle/nifi-framework/nifi-web/nifi-web-api -am -P contrib-check clean install — 0 Checkstyle violations, RAT license check passed, 642/642 tests passed, BUILD SUCCESS

Verification

Build

• Build completed using ./mvnw clean install -P contrib-check (scoped to nifi-framework-bundle/nifi-framework/nifi-web/nifi-web-api and its dependencies via -pl ... -am; full reactor build not run locally, deferred to CI) — 0 Checkstyle violations, RAT license check passed, 642/642 tests passed, BUILD SUCCESS
• JDK 21
• JDK 25 — not available (not offered by internal software catalog); project's maven.compiler.release targets 21

Licensing

• No new dependencies added
• N/A — no LICENSE/NOTICE changes required

Documentation

• N/A — no documentation changes in this PR

[abij]

@exceptionfactory I was building Grafana dashboards for NiFi Clusters and noticed 2 issues and 1 improvement. First of all so cool that this long-lived project is still so actively maintained. I have created JIRA-tickets and implementation of 3 separate small PR's (with some help of AI, less honor/pride to gain for myself...).

Please have a look and have a nice weekend.

[pvillard31]

Thanks for the contribution @abij - do you mind rebasing your PR as I merged the other one and there are now merge conflicts?