Update sampling algorithm to use random.random()

[mjbryant]

Addresses #8

[coveralls]

Coverage remained the same at 100.0% when pulling e451592 on mbryant-issue-8-sample-rate-over-50 into 1724d7d on master.

[kaisen]

🚢

[prat0318]

the rationale behind the previous logic was to make the selection deterministic. So, if we wanted to not change sample ratio we can chose a trace id such that it would result in selection.

If trace_ids are random, then it would lead to correct randomness in selections as well, but if they are not, then it might lead to what is seen in #8.

[kaisen]

@prat0318 heh, @mbryant and I talked about that particular reason during lunch :) It's an useful way of ensuring a particular trace_id is sampled (as long as the sampling rate stays constant) but there are better ways to force a trace now.

[prat0318]

++