Instrument service and client for end-to-end request/response tracking

[christophebedard]

This is similar to tracking messages from publisher to subscription (publication to callback). We want to be able to track a request from the publication/send on the client side to the take & callback on the service side, and similarly for responses.

Some tracepoints already exist:

• rcl_service_init
• rclcpp_service_callback_added
• rcl_client_init

And the following tracepoints are also used for services with AnyServiceCallback:

• rclcpp_callback_register
• callback_start/callback_end

The following tracepoints are missing:

• Request send
• Request take
• Response send
• Response take

To track a request/response from end to end, we would ideally use a combination of the client GID + sequence number. With pubs/subs, the publisher GID read from the publisher's side does not match the publisher GID on the subscription's side. However, this may work with clients/services, because the client GID value is sent as-is alongside the request/response; we do not have to rely on the underlying DDS implementation, which could be faulty (https://github.com/ros2/rmw_cyclonedds/issues/377).

Add tests in test_tracetools, and update the design document.

[christophebedard]

I'm currently working on this.