Add performance metrics

[Andrea Venuta]

It should be easy to add a performance metrics tracker for debugging purposes.

Things that should be tracked:

• Duration of message processing step
• Duration of Pipeline::prepare_render
• Duration of Pipeline::render
• Duration of client code frame generation
• Frame-to-frame elapsed time

These metrics don't necessarily need a big object, they can be stored in a struct of floats and updated via exponentially-weighted moving average with a low coefficient like 0.1 which should stabilize after 100 frames.