Backend: Total event counts metrics (not grouped by user)

Summary

Similar to #574400 (closed) which provides total event counts per user, we need an endpoint to provide total event counts not grouped by user - i.e., aggregate counts across all users for AI/Duo events.

Background

Issue #574400 (closed) provides metrics grouped by user via aiUserMetrics, allowing us to see event counts for individual users. However, we also need the ability to query aggregate event counts across all users without the user grouping.

Issue #570987 (closed) implements this for Duo Code Review specifically, but we need a general solution that works for all AI/Duo features.

Proposal

Add event count metrics to aiMetrics endpoint that provide aggregate counts across all users, similar to the structure being implemented in #570987 (closed):

aiMetrics(startDate: 'x', endDate: 'y') {
  codeSuggestions {
    codeSuggestionsAcceptedEventCount
    codeSuggestionsShownInIdeEventCount
    # ... other code suggestion events
  }
  duoChat {
    duoChatInteractionEventCount
    # ... other duo chat events  
  }
  codeReview {
    # ... code review events (from #570987)
  }
  # ... other AI features
}

Related Issues

• #574400 (closed) - Total event counts per user metrics (grouped by user)
• #570987 (closed) - Duo Code Review namespace-level metrics (specific to Code Review feature)