For certificates with leases vault.expire.num_leases this metric could be used for tracking the number of certificates. What can be used to track the number when lease generation is disabled? Also would be interested in knowing overall metrics available for pki certs. (Couldn’t get a lot of info here -Telemetry | Vault by HashiCorp)
See vault.route.create.<mountpoint> in that doc.
And be aware of the PKI parameters, ie, do you really need a short lived cert to have a lease in Vault (PKI - Secrets Engines - HTTP API | Vault by HashiCorp)