Add count of flows and agent chats executed each month to AI tracking dashboards

Problem

As a customer I want to get a basic understanding of how much Duo Agent Platform related features are being used in a specific project.

Desired Outcome

Add both a raw count of flows being executed (excluding chat sessions) as well as a count of chat sessions being executed per month in the Duo Usage dashboard in https://gitlab.com/groups/gitlab-org/-/analytics/dashboards/ai_impact as another line-item after Duo Chat Usage and Duo RCA usage.

Implementation Plan

1. Use the agent_platform_session_started event.
2. Implement new service to query data efficiently
3. Create separate counters:
   1. All events with flow_type = chat should count towards chat
   2. All other events should count to the count of flows.
4. Expose it to GraphQL under aiMetrics
5. Update FE to query new data

The FE should show the counts in this table: