Add merge_request branch exception bypass event

What does this MR do and why?

This MR adds a new event that is triggered whenever an MR is bypassed through the branch exceptions in security policies.

Local testing output

Monitored events or key path prefix: check_merge_request_branch_exceptions_bypass

RELEVANT METRICS

Key Path	Monitored Events	Instrumentation Class	Initial Value	Current Value
redis_hll_counters.count_distinct_value_from_check_merge_request_branch_exceptions_bypass_monthly	check_merge_request_branch_exceptions_bypass	UniqueCountMetric	0	1
redis_hll_counters.count_distinct_value_from_check_merge_request_branch_exceptions_bypass_weekly	check_merge_request_branch_exceptions_bypass	UniqueCountMetric	0	1

SNOWPLOW EVENTS

Event Name	Collector Timestamp	Category	user_id	namespace_id	project_id	plan	Label	Property	Value	Extra
check_merge_request_branch_exceptions_bypass	1756295968737	MergeRequests::ReopenService		249	63	ultimate			259	{}

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Create a group or a project
• Create MR approval policy with branch exceptions:

approval_policy:
  - name: Branch Bypass
    description: ''
    enabled: true
    rules:
      - type: any_merge_request
        branch_type: protected
        commits: any
    actions:
      - type: require_approval
        approvals_required: 1
        role_approvers:
          - maintainer
          - developer
      - type: send_bot_message
        enabled: true
    approval_settings:
      block_branch_modification: true
      prevent_pushing_and_force_pushing: true
      prevent_approval_by_author: true
      prevent_approval_by_commit_author: true
      remove_approvals_with_new_commit: true
      require_password_to_approve: false
    fallback_behavior:
      fail: closed
    bypass_settings:
      branches:
        - source:
            pattern: hotfix*
          target:
            name: main

• Execute the internal events monitor:

bundle exec rails runner scripts/internal_events/monitor.rb check_merge_request_branch_exceptions_bypass

• Create a project and create an MR with source branch hotfix-123
• Verify that the metric is updated with the MR ID

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #549575 (closed)