Tracking progress towards completion.

Is there a way to estimate "progress towards completion" in the context of either timely or differential? Let's say we pushed some updates to DD, advanced the timestamp and are now calling step_or_park in a loop waiting for the probe to report that all updates have successfully propagated through the pipeline. This can take a long time, e.g., when populating the pipeline with initial data or when processing a small update that triggers large amount of recomputation. In these situations it may be nice to comfort the user with a little progress bar showing approximately how much longer they have to wait.

@frankmcsherry, do you happen to have some magic up your sleeve that might help here? :)

@Kixiron, @RDambrosio016.

Start by reading the timely and differential APIs around step_or_park and timestamp advancement. Determine whether the pipeline exposes enough information to estimate propagation or recomputation progress, then define what an approximate progress indicator should report for initial loads and small updates. Done would require an agreed design and an implementation path, but no files or tests are named.