Separating formatters from exporters

[thefotios]

Hi, I started using this for some projects yesterday and definitely love it. I needed to make a few fixes (which I'll be submitting PRs for this weekend), but this definitely fits into it. I had 2 ideas I wanted to run by you before embarking on them.

I think it would make sense to separate the formatting from the exporting. That would allow flexibility to say "format this as Zipkin, but print to the screen/file". We could make this transparent to the user by each exporter having a default formatter and also ensuring compatibility (eg, the StackDriver exporter can only use the StackDriver formatter).

Down the road, this could allow a much simpler pattern for different endpoints to be totally separate subpackages/plugins for each exporter (eg, you would explicitly pip install opencensus-python[stackdriver]).

[liyanhui1228]

Hi @thefotios, thanks for opening this issue! Your idea for creating a formatter class and separating the logic of formatting spans for different exporters sounds good to me, and appreciate it if you have started working on it. And for separating subpackages/plugins, this is also worth doing. But restructure the distribution model is somewhat a big change, let's work on separating the formatters first and then see how we could make subpackages for stackdriver.