Expose Metadata.size

[He-Pin]

Motivication:

I want to get the metadata's size, but currently I can only get it with metadata#keys#size, access it directly will reduce some overhead.

NOTE:
It was reverted in PR: #3008

My currently usage:

        //https://github.com/grpc/grpc-java/pull/3008
        if (!headersSent && !extraHeaders.keys().isEmpty()) {
            //trailers.merge(extraHeaders);
            sendHeaders(new Metadata());
        }