Instrument batch building

[Mirko-von-Leipzig]

Instrument batch building in a similar fashion to the block building in #675.

I recommend starting by defining the stages of the pipeline as you would want to explore them. As in, what timing information are you interested in - this defines your top-level spans. Likely this won't quite fit with the code; iterate until you are at a reasonable compromise between code clarity and trace generation.

[bobbinth]

I think we can tackle this after #583.