the zipkin_span decorator may allow too many arguments

[ericchu94]

@mjbryant says in #21:

Should we be disallowing certain passed-in arguments in this form? In my mind, the decorator form doesn't make any sense for zipkin_attrs or annotations, as these should change on a per-call basis. You're probably never going to want to log span after span with, say, the same trace_id, or the same timestamped annotations.

We could check for self.zipkin_attrs and raise an exception if a zipkin_span with these set is used in a decorator fashion.

Just a thought.

The notable arguments are zipkin_attrs and annotations. We should investigate which arguments are intended to be used with the zipkin_span decorator, and see if we can do something about it