Part of [#711] Add the `SumObserver` instrument, the asynchronous version of a `Counter`. Refer to the [API specs](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/metrics/api.md#sumobserver).
Part of [#711]
Add the
SumObserverinstrument, the asynchronous version of aCounter.Refer to the API specs.