function getHostname(method, methodDescriptor) {
  // method = hostname + methodDescriptor.name(relative path of this method)
  return method.substr(0, method.length - methodDescriptor.name.length);
}
Get the hostname of the current request.
@template REQUEST, RESPONSE
@param {string} method
@param {!MethodDescriptor<REQUEST,RESPONSE>} methodDescriptor
@return {string}