Add third, undeterminated state to the result labels

[mellowagain]

Migrated from #85 (comment)

In the spec draft v1.0.0 only two result label values are accepted: ok and error

My suggestions would be to have a third, undeterminated state (e.g n/a). This would allow consumers of the library (e.g Dora) to:

• Guide users to add correct function success/failure, if the library fails to automatically do so
• Warn users if they have SLOs that depend on these functions

This would necessitate a breaking change in the spec as the current spec draft requires functions, for which a result cannot be determinated, to be labeled as ok.

[flenter]

This might not be the best place for this, but I'm more struggling with coming up with cases where autometrics would be unable to determine whether something was executed successfully. What sort of situations would that be?