election observe sometimes lose the first event

[tunefun]

          @lburgazzoli @giri-vsr hello, it seems that election observe have the same problem

jetcd/jetcd-core/src/main/java/io/etcd/jetcd/impl/ElectionImpl.java

Lines 116 to 119 in 85a9a54

	stub.observe(request)
	.handler(value -> listener.onNext(new LeaderResponse(value, namespace)))
	.endHandler(ignored -> listener.onCompleted())
	.exceptionHandler(error -> listener.onError(toEtcdException(error)));

and maybe we can also registers end handler before watch request？

Originally posted by @tunefun in #1261 (comment)

[lburgazzoli]

a pr and related integration test would be more than welcome

[tunefun]

a pr and related integration test would be more than welcome

I think this issue same as origin issue, so I didn't describe too much. I will try to present a pr and integration test