Emit more detailed trigger usage metrics

E.g. we emit metrics by binding type / direction, but we don't have a way to discover how many webhook functions there are. We emit http metrics, but webhooks are just a flavor of http.

The issue names no files, tests, or entry points. Start by locating the existing binding-type, direction, and HTTP metric emission, then determine how webhook function counts and webhook-specific usage should be defined; done requires an agreed metric design and coverage for the requested measurements.

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