Histogram Metrics: Add some default boundaries or algorithms to Histogram

[gavindoudou]

PR #272 creates a oneof for histogram buckets, I wonder:

1. Do we have a plan to implement some default boundaries for receivers? It would be convenient for exporters if we have some standards instead of letting developers to set themselves. Maybe we could add one option to make it compatibility with Prometheus:
   DefBuckets = []float64{.005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5, 10}
2. Do we have a plan to add sketch options to the histogram? As discussed here: Metrics Histogram instrument default: any histogram sketch opentelemetry-specification#982, fixed boundary is the solution now. I think we still need the sketch options to have precise histogram.

[yzhuge]

See open-telemetry/oteps#149