Make metadata and callback optional

[shaxbee]

No description provided.

[stanley-cheung]

Thanks for the contribution. I suggest we hold off on this for now. In the gRPC Node API, the callback is mandatory. We are trying to keep the surface API as consistent as possible.

[shaxbee]

@stanley-cheung I've removed callback optionality.

[stanley-cheung]

How does that work though? In unary calls, it's

client.sayHello(request, metadata, callback);

How do you make metadata optional but callback required?

Also unfortunately, changing abstractclientbase.js is a too much of a risk. There are some internal code that will need to make a coordinated change. I will certainly take this into consideration in a future refactor.

[shaxbee]

Optional as in it can be nullable. This is consistent with nodejs grpc package API.

Only change in abstractclientbase.js is this:

if (metadata) {
  xhr.headers.addAll(metadata);
}