Add the top-level metrics API package. See: - The [spec](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/metrics-api.md) - [Metrics work in OpenCensus](https://github.com/census-instrumentation/opencensus-python/issues?utf8=✓&q=label%3Ametrics) - [OT metrics RFC](https://github.com/open-telemetry/rfcs/pull/4)
