Support health and liveness probes

[j-vizcaino]

I'm adding a liveness probe to a manager built with controller-runtime.
The manager already leverages the HTTP service provided natively to serve Prometheus metrics.

I would like to add another endpoint /live to provide the liveness of the application, on the same HTTP server that is used for metrics.
There does not seem to be a way to do this today because the http mux is created internally and attached to the HTTP server.

A way to circumvent this is to basically copy/paste the http server logic into a package of my own and add the extra handler but it feels sad that there is no way to reuse the existing code.

[DirectXMan12]

we should have a general liveness hook. I'm not sure we want to make this a general HTTP serving platform, but liveness support would definitely be useful.

/kind feature
/priority important-longterm

[j-vizcaino]

Agreed, having a standard way for serving a liveness hook would be nice.
At the very least, it would be useful to be able to provide the HTTP server somehow as that would let the user tweak and reuse that component elsewhere.

[DirectXMan12]

/good-first-issue

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[GrigoriyMikhalkin]

Hi, i would like to try to implement this.

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