*: initialize gRPC server metrics with zero values

[brancz]

This initializes the gRPC metrics to their respective zero-values. This is useful for discovering metrics, as well as being able to differentiate whether metrics are missing due to the target missing or currently just no request having generated the metrics yet.

@xiang90 @gyuho

[gyuho]

lgtm. Confirmed that now all gRPC metrics show up in /metrics endpoint.

Thanks!

[xiang90]

lgtm

[xiang90]

@gyuho do you know why test-cov is failing?

[gyuho]

@xiang90

Too many goroutines running after all test(s).
1 instances of:
github.com/coreos/etcd/etcdserver/api/v3rpc.(*serverWatchStream).recvLoop(...)
	github.com/coreos/etcd/etcdserver/api/v3rpc/_obj/watch.go:284 +0x1db
github.com/coreos/etcd/etcdserver/api/v3rpc.(*watchServer).Watch.func2(...)
	github.com/coreos/etcd/etcdserver/api/v3rpc/_obj/watch.go:129 +0x35
created by github.com/coreos/etcd/etcdserver/api/v3rpc.(*watchServer).Watch
	github.com/coreos/etcd/etcdserver/api/v3rpc/_obj/watch.go:127 +0x2f9
exit status 1
FAIL	github.com/coreos/etcd/clientv3/integration	220.750s

Will create an issue.

[gyuho]

Created a separate issue #8883.