async cancellation should use same attribute name

[anuraaga]

Currently the attribute name when canceling an async return value via WithSpan encodes the instrumentation name as well

guava.canceled
reactor.canceled

I would expect them all to be the same. Though it's true that because the async end strategy is not the instrumentation name (which is otelannotations), it would lose that information.

[trask]

some ideas:

• future.canceled
• promise.canceled
• async.canceled

I don't love async.canceled because async has such a broad meaning. I think my vote currently would go toward promise.canceled.

[trask]

Closing, these attributes are now opt-in under an experimental flag, can revisit later if anyone has renewed interest.