diagnostic channel events on body consume / cancel

Discussed in https://github.com/nodejs/undici/discussions/1170

^{Originally posted by artur-ma January 19, 2022}
Following this https://github.com/DataDog/dd-trace-js/pull/1642#issuecomment-1009298056
Looks like to have full visibility and have good APM instrumentation, the APM agent should know when the request actually finished, including reading the body from the socket.

Do you have any thoughts or ideas on what is the right way to implement it?

AFAIK the body object always be consumed or cancelled according to the docs, so the right direction to look on is the Readable object

Specifically these parts:

https://github.com/nodejs/undici/blob/main/lib/api/readable.js#L41-L56 - Destroy
https://github.com/nodejs/undici/blob/main/lib/api/readable.js#L266-L283 - ConsumeFinish

IMO just sending an event from "ConsumeFinish" is not enough since it will need some reference on the request itself to be able to finish the right span which currently is missing on Readable

Start with the linked discussion and the referenced sections of lib/api/readable.js, especially Destroy and ConsumeFinish, then inspect how the request and body are connected. The issue does not define an event shape or acceptance criteria; completion would require agreeing on how diagnostic events identify the request and cover body consumption or cancellation.