add new metric that mirrors the state of the health check in EPP

[k15r]

Description

EPP availability is currently based on kube-state-metrics. To remove this dependency expose a new metric
eventing-epp-health. This metric will be set to the same value as the regular health endpoint.
In fact this means at the moment that we will set it to 2xx during startup and not change it, as this is the exact behaviour of the health endpoint. In alert queries we can then check if the value is present or not.

Acceptance

• expose new metric eventing-epp-health