Right now, AttributeValues in span tags are supposed to be one of (int, bool, float), but we do no enforcement of that value when added.

This then requires error handling to be put into the exporters themselves, which will become fairly redundant: https://github.com/open-telemetry/opentelemetry-python/pull/320/files/93331ff73905ab0dad11110bdac45737098c2d26#r360226796

It would be great if we could enforce the types of attributes when added instead.