class_docstr: This class constructs a protocol service class using a service descriptor.
  
  Given a service descriptor, this class constructs a class that represents
  the specified service descriptor. One service builder instance constructs
  exactly one service class. That means all instances of that class share the
  same builder.
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docstr: Calls the method described by a given method descriptor.
  
  Args:
    srvc: Instance of the service for which this method is called.
    method_descriptor: Descriptor that represent the method to call.
    rpc_controller: RPC controller to use for this method's execution.
    request: Request protocol message.
    callback: A callback to invoke after the method has completed.
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code: def _CallMethod(
      self, srvc, method_descriptor, rpc_controller, request, callback
  ):
      """Calls the method described by a given method descriptor.
  
      Args:
        srvc: Instance of the service for which this method is called.
        method_descriptor: Descriptor that represent the method to call.
        rpc_controller: RPC controller to use for this method's execution.
        request: Request protocol message.
        callback: A callback to invoke after the method has completed.
      """
      if method_descriptor.containing_service != self.descriptor:
          raise RuntimeError(
              "CallMethod() given method descriptor for wrong service type."
          )
      method = getattr(srvc, method_descriptor.name)
      return method(rpc_controller, request, callback)
  
label_desc: Use image node_id 3 for calling the _ServiceBuilder obj's underlying member method code with example usage: obj._CallMethod(srvc, method_descriptor, rpc_controller, request, callback) and returns: method