Adapt to telemetry span exception event name convention

[binaryseed]

The telemetry:span convention specifies that :exception is used in exception cases instead of :stop events... This PR simply corrects the event name for exception cases.

• Add telemetry:span/3 beam-telemetry/telemetry#57

@whatyouhide @keathley @GregMefford @josevalim

[whatyouhide]

Fixing a typo through GH suggestions, let's see if I can commit those myself :)

[binaryseed]

oops, at least I was consistently misspelling it!

[GregMefford]

Wait... this shouldn’t have been an :exception because it’s not representing an early exit of the process that would have a stacktrace attached; it’s proceeding to completion but returning an error. This is why we decided to call that event :exception instead of :error or :failure, right?

[josevalim]

Yes, @GregMefford , good catch. We should only use exception if the caller is raising. We should probably add telemetry:span/3 to the lib itself, otherwise we will have mistakes like this popping up. Instead we should have status field for "soft" failures.

[binaryseed]

Oh, my bad - I did not realize that. I saw the keys kind and reason being used and thought that :exception was meant to be broader than what you are saying..

@GregMefford Do you think we could sketch out a tiny example of code that would make the distinction between the 2 cases clearer? I didn't really gather that from all the descriptions & conversations we've been having, so it's possible others might make the same mistake.

Sorry for the churn here!!

[whatyouhide]

For now, I'm going to revert this. :)