Implement metric observer and callbacks for gauges. Part of [#155] Depends on [#160]. [RFC](https://github.com/open-telemetry/oteps/blob/master/text/0008-metric-observer.md) [Specs](https://github.com/open-telemetry/opentelemetry-specification/pull/250)
Implement metric observer and callbacks for gauges.
Part of [#155]
Depends on [#160].
RFC
Specs