Add metrics for SAST False Positive workflow tracking

Summary

Add comprehensive metrics to track user interactions with the SAST False Positive workflow to understand usage patterns and effectiveness.

Metrics to Track

Core Workflow Metrics

• FP Workflow Triggered: Count when a vulnerability enters the FP workflow
• FP Label Applied: Count when a user labels a vulnerability as a false positive
• FP Label Removed: Count when a user removes the false positive flag from a vulnerability

Additional Context Metrics

• Vulnerability severity when FP workflow is triggered
• Time spent in FP workflow before decision
• Source of FP workflow trigger (manual vs AI suggestion)
• User role/permissions when interacting with FP workflow

Implementation Requirements

• Use GitLab's internal event instrumentation framework
• Ensure metrics are anonymized and privacy-compliant

Acceptance Criteria

• Metrics are instrumented for FP workflow trigger events
• Metrics capture FP label application events
• Metrics capture FP label removal events
• Events include relevant context (severity, source, etc.)
• Metrics follow GitLab's instrumentation standards
• Documentation updated for new metrics