feat: Covered Experience logging

Part of Implement Covered Experience event emission in ... (gitlab-com/gl-infra/observability/team#4114 - closed).

Adds log messages to the Covered Experience events.

Log message examples:

{
    "severity": "INFO",
    "time": "2025-08-01T13:40:38.688Z",
    "checkpoint": "start",
    "covered_experience": "testing_sample",
    "feature_category": "source_code_management",
    "urgency": "sync_fast",
    "start_time": "2025-08-01 13:40:38 UTC",
    "elapsed_time_s": 0.0,
    "urgency_threshold_s": 2
}

{
    "severity": "INFO",
    "time": "2025-08-01T13:40:38.605Z",
    "meta.user_id": 123,
    "meta.project_id": 456,
    "correlation_id": "f85a43fc4d9040ab6d8c3bb8a45a4b6c",
    "checkpoint": "intermediate",
    "covered_experience": "testing_sample",
    "feature_category": "source_code_management",
    "urgency": "sync_fast",
    "start_time": "2025-08-01 13:40:38 UTC",
    "checkpoint_time": "2025-08-01 13:40:38 UTC",
    "elapsed_time_s": 0.000221,
    "urgency_threshold_s": 2,
    "step": "validation",
    "items_processed": 50
}

{
    "severity": "INFO",
    "time": "2025-08-01T13:40:38.677Z",
    "checkpoint": "end",
    "covered_experience": "testing_sample",
    "feature_category": "source_code_management",
    "urgency": "sync_fast",
    "start_time": "2025-08-01 13:40:38 UTC",
    "end_time": "2025-08-01 13:40:38 UTC",
    "elapsed_time_s": 0.000181,
    "urgency_threshold_s": 2,
    "error": true,
    "error_message": "#<RuntimeError: Something went wrong>"
}