Context

Clients cannot get a list of the services a certain server is watching to and their respective status.

In the context of Kubernetes, we have livez and readyz endpoints, by listing all the services a server is checking, we could return a response like the /readyz?verbose Kubernetes API returns:

[+]ping ok
[+]log ok
[+]etcd ok
[+]poststarthook/start-kube-apiserver-admission-initializer ok
[+]poststarthook/generic-apiserver-start-informers ok
[+]poststarthook/start-apiextensions-informers ok
[+]poststarthook/start-apiextensions-controllers ok
[+]poststarthook/crd-informer-synced ok
[+]poststarthook/bootstrap-controller ok
[+]poststarthook/rbac/bootstrap-roles ok
[+]poststarthook/scheduling/bootstrap-system-priority-classes ok
[+]poststarthook/start-cluster-authentication-info-controller ok
[+]poststarthook/start-kube-aggregator-informers ok
[+]poststarthook/apiservice-registration-controller ok
[+]poststarthook/apiservice-status-available-controller ok
[+]poststarthook/kube-apiserver-autoregistration ok
[+]autoregister-completion ok
[+]poststarthook/apiservice-openapi-controller ok
healthz check passed

Use case

This is not only useful for having status reports like Kubernetes readyz endpoints; it also allows services to simplify the set up when creating microservices that publish their status to status page systems such as:

https://cachethq.io/
https://www.atlassian.com/software/statuspage

Change

This PR introduces a new List endpoint to the Health service in order to list all the services that a certain server is watching to.

Alternative solution

We could extract the List endpoint into a new service to avoid breaking the v1 API. A new HealthLister (or any other name) must be created.

Proposal

grpc/proposal#468