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: Returns the class of the request protocol message.
  
  Args:
    method_descriptor: Descriptor of the method for which to return the
      request protocol message class.
  
  Returns:
    A class that represents the input protocol message of the specified
    method.
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code: def _GetRequestClass(self, method_descriptor):
      """Returns the class of the request protocol message.
  
      Args:
        method_descriptor: Descriptor of the method for which to return the
          request protocol message class.
  
      Returns:
        A class that represents the input protocol message of the specified
        method.
      """
      if method_descriptor.containing_service != self.descriptor:
          raise RuntimeError(
              "GetRequestClass() given method descriptor for wrong service type."
          )
      return method_descriptor.input_type._concrete_class
  
label_desc: Use image node_id 4 for calling the _ServiceBuilder obj's underlying member method code with example usage: obj._GetRequestClass(method_descriptor) and returns: method_descriptor