chore: add evaluation event count metrics to subscriber

[kentakozuka]

This pull request includes changes to improve the metrics collection for evaluation events in the subscriber processor. The most important changes include adding a new counter for evaluation events and updating the incrementEvaluationCount method to increment this new counter.

Metrics collection improvements:

• pkg/subscriber/processor/metrics.go: Added a new constant appVersion to be used as a label in the new counter.
• pkg/subscriber/processor/metrics.go: Introduced a new evaluationEventCounter using prometheus.NewCounterVec to count evaluation events with labels for tag, feature_id, and app_version.
• pkg/subscriber/processor/evaluation_events_evaluation_count_event_persister.go: Updated the incrementEvaluationCount method to increment the evaluationEventCounter with appropriate label values.

[kentakozuka]

@cre8ivejp Fixed 👍