Utilize is_recording instead of Defaultspan for instrumentations if sampled

[lzchen]

If a span is determined to be not sampled returned from the sampler, a DefaultSpan implementation is returned. This was beneficial because we could use the same code for all span related logic but have different behaviour whether it is a real span or a Defaultspan. However, we will be having performance losses using the mechanism for instrumentations, in which we call all the methods and set all attributes on the span (even if they do nothing) -> see requests. Most other languages check the "is_recording" flag and do not run any of the span related logic if not recording. We should do the same.