Backend: Add Service Ping metrics for AI Catalog

About

We should add AI Catalog to the service ping metrics.

These are snapshot counts of data on GitLab, sent weekly from other GitLab instances.

See https://docs.gitlab.com/development/internal_analytics/metrics/metrics_instrumentation

Proposal

Description	Example scope
Count of flows enabled for projects	ItemConsumer.where(enabled: true).where.not(project: nil)
Count of flows enabled for groups	ItemConsumer.where(enabled: true).where.not(group: nil)
Count of public flows in AI Catalog	Item.flow.where(public: true)
Count of private flows in AI Catalog	Item.flow.where(public: false)
Count of public agents in AI Catalog	Item.agent.where(public: true)
Count of private agents in AI Catalog	Item.agent.where(public: false)