Clarify the API on behavior of asynchronous callback reentrancy, timeout, duplicate observations

[jmacd]

What are you trying to achieve?

The current specification is vague on what happens if an asynchronous instrument callback times out, on what happens if the same SDK or different SDK needs to call invoke the callbacks before another collection finishes. Also, requirements are written that impact the user, so the API specification should be phrased as a requirement to document what is expected of the user, not specify the user's behavior.

What did you expect to see?

More rigor on what SDKs should do, what kind of reentrancy guarantees should be expected for callbacks.

Additional context.

Questions arose in #2317:

#2317 (comment)
#2317 (comment)
#2317 (comment)

cc/ @dyladan @legendecas @anuraaga