Proposal for C++ based ros2 topic hz

Hello, I am considering putting together an MR to contribute a more performant C++ implementation of ros2 topic hz (and maybe ros2 topic bw while we are at it).

I think even with #1005 merged, performance of ros2 topic hz remains poor, especially for high frequency topics. I think #843 is still an issue. I did some informal experiments awhile ago to confirm this, which I can replicate and share if desired (did anyone else run experiments on this btw? would be good to compare notes)

When I switch to a C++ based implementation I see a ~8x reduction in CPU usage, and can accurately report topic frequencies up to 10Khz, whereas the existing ros2 topic hz caps out around a 2-3khz.

There are two caveats to this that I see already.

1. Using C++ from the python topic verb will require pybind11 or ctypes, which may complicate distribution.
2. the --filter option accepts arbitrary python code, so likely we would want to fallback to the original hz.py implementation just for --filter, which is a little ugly.

I'd like to get the maintainers thoughts on this, would this MR be something you are interested in merging? Are there other gotchas I am not considering?

Start with ros2topic/ros2topic/command/topic.py and the existing hz.py implementation, then reproduce the reported high-frequency performance measurements. Clarify the maintainer-approved design for the C++ integration, distribution approach, and --filter fallback; done would mean an accepted implementation with comparable performance evidence and coverage for the affected commands.