Automatic merge from submit-queue

Added a basic monitor for providing etcd version related info

Fixes #41071 

This tool scrapes metrics partly from etcd's /version and /metrics endpoints and partly using etcdctl and exposes them as prometheus metrics at `http://localhost:9101/metrics` endpoint on the master. Here is a summary of the metrics it exposes (self-explanatory from the code):
-        etcdVersionFetchCount   = prometheus.NewCounterVec(
                prometheus.CounterOpts{
                        Namespace: "etcd",
                        Name: "version_info_fetch_count",
                        Help: "Number of times etcd's version info was fetched, labeled by etcd's server binary and cluster version",
                },
                []string{"serverversion", "clusterversion"})
-         etcdGRPCRequestsTotal   = prometheus.NewCounterVec(
                prometheus.CounterOpts{
                        Namespace: namespace,
                        Name: "grpc_requests_total",
                        Help: "Counter of received grpc requests, labeled by grpc method and grpc service names",
                },
                []string{"grpc_method", "grpc_service"})

For further info on how to run this as a binary/docker-container/kubernetes-pod and checking the metrics, have a look at the README.md file.

cc @fgrzadkowski @wojtek-t @piosz