[FLINK-38787] Introduce FlinkBlueGreenDeployment Metrics

[james-kan-shopify]

What is the purpose of the change

Jira Issue: https://issues.apache.org/jira/browse/FLINK-38787

This pull request adds lifecycle metrics support for FlinkBlueGreenDeployment resources, enabling operators to monitor blue-green deployment state transitions and timing. This provides observability into the deployment pipeline, helping identify bottlenecks and track deployment health. The implementation heavily mirrors the existing FlinkDeployment metrics' implementation to ensure consistency.

Note: Most lines of code introduced are for test files!

Brief change log

1. Real-time State Distribution Tracking

   • Namespace-level gauges showing current count of deployments in each blue-green state and Flink job status.

2. Lifecycle Transition Timing

   • Histogram metrics measuring duration of key transitions (initial deployment, blue-to-green, green-to-blue) and time spent in each state, available at system and namespace levels.

3. Historical Failure Tracking

   • Accumulating counter that increments on each transition to FAILING state, enabling failure rate calculation and long-term reliability monitoring.

Verifying this change

This change added tests and can be verified as follows:

• Added BlueGreenLifecycleMetricsTest to verify histogram creation, namespace isolation, and metric registration

• Added BlueGreenResourceLifecycleMetricTrackerTest to verify state transition timing, rollback scenarios, and intermediate state recording

• Tests cover initial deployment, blue-to-green transitions, green-to-blue transitions, and failed transition rollbacks

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no

• The public API, i.e., is any changes to the CustomResourceDescriptors: no

• Core observer or reconciler logic that is regularly executed: no

Documentation

• Does this pull request introduce a new feature? yes

• If yes, how is the feature documented? docs