Feature request

Feature description

The auto-populated seq field was dropped from Header in transition from ROS1 to ROS2 (ros2/common_interfaces#2). This data was useful in measuring lost messages, and I imagine will be even more helpful with ROS2's focus on unreliable communication channels.

Implementation considerations

Some implementation hints were provided in https://discourse.ros.org/t/foxy-message-api-review/13105/17