found what I was after, it is the `bitswap` system, or more specifically the `Bitswap.TaskWorker.Work` event.

Here is an example log from that even (hashes omitted)
```
{"Block":"Qm....","ID":1,"Target":"Qm.....","event":"Bitswap.TaskWorker.Work","session":".....","system":"bitswap","time":"2019-05-01T05:42:14.62936035Z"}|
```

I'm guessing `Block` is the particular block in question, however I'm not certain what `Target` is?? Is `Target` the complete dag the block is a part of