Add dashboard for Gatekeeper usage metrics

[natsumesou]

As an admin, I would like to see metrics on how frequently and heavily each Gatekeeper (connector) is used, so that I can understand adoption and usage patterns across the deployment. Currently there is no dashboard that surfaces this information.

• Show usage frequency/volume per gatekeeper and vendor
• Show trends over time (e.g., daily/weekly/monthly)
• Highlight the most/least used gatekeepers

[natsumesou]

#302 (comment)