Added diagnostics to diff_drive_controller

[sandeepdutta]

Hello All,

I have added a feature in diff_drive_controller to publish "diagnostics" messages, This helped me immensely in debugging my robot. I would to like feed this back in the hope it will help others debug their robots. The code is available at https://github.com/sandeepdutta/diff_drive_controller , derived from the humble branch.

Thank you Sandeep

[christophfroehlich]

Hi. Thanks a lot for your code example.

I haven't used diagnostics messages in ROS 2, and it doesn't seem to be very common having in mind that there isn't even a single mention on docs.ros.org But as we have this inside the controller manager as well, I guess it's adding value to the controllers as well.

Would you mind creating a PR with your addition targeting the rolling branch?