ProgressEvent's messages and internal fields

Hi Frank!

For profiling progress & visualizing dataflow frontiers, I believe I need some information in ProgressEvents on what kind of progress actually happened. I think that the messages and internal fields could help me with that [1].

Currently they are only mocked up in send(...) and receive(...). I'd be happy to create a PR and implement them, if you can guide me a little on what the Vec fields are supposed to be. E.g., what are target and source descriptor in this context (I assume they are the first two usize coordinates in the Vec's tuples)?

Cheers,
Malte

[1]

pub struct ProgressEvent {
    ...
    /// List of message updates, containing Target descriptor, timestamp as string, and delta.
    pub messages: Vec<(usize, usize, String, i64)>,
    /// List of capability updates, containing Source descriptor, timestamp as string, and delta.
    pub internal: Vec<(usize, usize, String, i64)>,
}

Start by tracing ProgressEvent and the send(...) and receive(...) paths, where the messages and internal fields are currently mocked up. Clarify the meanings of the Vec tuple coordinates, including source and target descriptors, before implementing the updates; done means these fields accurately report message and capability progress for profiling.