proposal for Views in the SDK

[tsloughter]

This proposal has each instrument associated with a default view when created and views replace the default aggregation associated with an instrument.

[jkwatson]

I had a couple of questions, but I generally like this. What I really like about it is that it can be implemented in 2 phases. 1) implement the default view support, and 2) implement full view support. The default view support maps reasonably well to my proposal in #126 (aside from the temporality question), but then can be built upon for full views.

[tsloughter]

@jkwatson yup, exactly, and I see implementing the default view support as more of a refactor of existing metric SDKs and not necessarily much new code.