API: decide what to do for invalid arguments

[fbogsany]

I'm not sure what we should do for invalid arguments in the minimal implementation, and am somewhat torn on the issue.

I just wanted to point out that there is a proposal currently in PR form here: open-telemetry/opentelemetry-specification#153.

There seem to be various opinions as you read through the comments. This comment proposed a somewhat pragmatic approach of falling back to a default when an argument is invalid. Quote (from the comment)

Specific case: I do think using a null name for Span could fallback to using a default name, instead of throwing. Documenting it is nice, but providing defaults for these kind of things is a good alternative ;)

It might be too early to make a final call on how to deal exceptions in the minimal implementation, but we should keep an eye on that PR and the specification repo for guidance.

Originally posted by @mwear in https://github.com/open-telemetry/opentelemetry-ruby/diffs

[fbogsany]

We settled on removing most runtime checks #115 and adding a "strict mode" #126