This is a ticket to track work related to the service introspection feature proposed in ros-infrastructure/rep#360

The high-level idea is to introduce service event messages that are published whenever a request or response is sent or received. Having service events available from topics makes it easy for users to inspect data flowing between services, aiding in debugging and validation. Please refer to the REP for details.

Here's a roadmap to implement the feature:

• Add new event message types in code generation for services
  • Add service_msgs package (Add service_msgs package rcl_interfaces#143)
  • Add dependency on service_msgs in rosidl_defaults for convenience ([Service introspection] service_msgs dependency rosidl_defaults#24)
  • Additions to code generation packages:
    • [service introspection] generate service_event messages rosidl#700
    • Service introspection rosidl_typesupport_fastrtps#92
    • Service introspection rosidl_typesupport#127
    • Service introspection rosidl_python#178
• Integration with client libraries
  • Add event message publishers to rcl services and clients (Service introspection rcl#997)
  • rclcpp (Service introspection rclcpp#1985)
  • rclpy (Service introspection rclpy#988)
  • demos (Service introspection demos#602)
• Add support to ros2service to echo service requests and responses ([service introspection] ros2 service echo ros2cli#745)
• Update existing tutorials (or add a new tutorial) related to services to use the new feature (Service Introspection Documentation and Tutorial ros2_documentation#2981)

Related to ros2/rosbag2#773 ros2/rosbag2#1063