Allow diagnostics to be published from a hardware_interface

[Timple]

Is your feature request related to a problem? Please describe.
For an actuator, often the actuator reports more values that just the measured one. For example current power consumed. It would be nice to have this information available on the /diagnostics topic.

Describe the solution you'd like
A way to easily supply diagnostics from a hardware_interface. For example:

hardware_interface::return_type MyActuator::read(const rclcpp::Time &, const rclcpp::Duration &);

hardware_interface::return_type MyActuator::write(const rclcpp::Time &, const rclcpp::Duration &);

hardware_interface::return_type MyActuator::produce_diagnostics(diagnostic_updater::DiagnosticStatusWrapper &stat);

Or:

hardware_interface::CallbackReturn on_configure(const rclcpp_lifecycle::State & previous_state);

hardware_interface::CallbackReturn add_diagnostic_task(const DiagnosticTask * t);

Describe alternatives you've considered
Creating a diagnostic_updater_ class member. Which also needs a node_ and then spin an executor. Like this: https://github.com/nicklgw/bdrive_hardware_interface/blob/f557bf1111f6ecc2df0389d25c7e73225e294a61/src/bdrive_hardware_interface.cpp#L93

Additional context

[github-actions]

This issue is being labeled as stale because it has been open 45 days with no activity. It will be automatically closed after another 45 days without follow-ups.

[Timple]

Do any of the maintainers have a preferred way of implementing this?

[saikishor]

Do any of the maintainers have a preferred way of implementing this?

Hello @Timple

We are planning to address this exact same thing in a Google Summer of Code project. Hopefully, we will have something soon. Your approach works for actuator interfaces, but not system interfaces with multiple joints.

[soham2560]

@saikishor could you expand on why this won't work for system interfaces with multiple joint?

[saikishor]

@saikishor could you expand on why this won't work for system interfaces with multiple joint?

Because one instance can carry information of only one information level, and for the System we might have multiple actuators or sensors and if we need multi-level diagnostics or some flexibility, a single wrapper wouldn't be enough :). I hope this answers your question @soham2560