Implement prometheus metrics endpoint

[drwatsno]

Hello

Prometheus metrics format is the de facto industry standard. We are using kratos in production (k8s) and we faced the problem of missing this feature (metrics endpoint with prometheus metrics)

Soultion
Implement metrics manager (just like in hydra), metrics endpoint, re-use code from Hydra
where possible

The next step could be to implement the OpenMetrics protocol (when ready)