Add a health check endpoint to the api-gateway-controller

[skhalash]

Description

Add a health check endpoint to the api-gateway-controller.

Reasons

A new way of monitoring Kyma runtime availability requires readiness probes to be configured for all workloads (deployments, stateful sets, daemon sets). When assessing Kyma components we realised that most of the self-written controllers/operators are missing health endpoints (which could be used as readiness probes).

As a temporary workaround we decided to abuse the /metrics endpoint for this purpose. But in the future it would useful to have a native health check endpoint.

Attachments

Here's an example from the nats operator which we could draw inspiration from.

[skhalash]

As I wrote above #11730 is a temporary solution and should be replaced by a real health check endpoint.