Provide a readiness check endpoint

[negz]

What problem are you facing?

xgql currently has support for metrics and tracing, but there's no simple way to detect whether the xgql pod is in a fundamentally broken state. The most common reason xgql might be completely non-functional would be because it can't reach the API server.

How could xgql help solve your problem?

xgql could add a readiness endpoint (e.g. /ready) that returned 200 OK if some basic sanity checks passed, e.g. that the Kubernetes API server was reachable.