Opentelemetry support for synchronous Gauges

[tauseef-ah]

Why doesnt OpenTelemetry provide support for Synchronous Gauges?
Although asynchronous seems efficient to handle the sampled value, there would still exist use cases where-in the observable sample value is provided by a library, and it would have been easier to push the value to Metric collector, then to save the same value and provide it when the callback function is called.

[jmacd]

See #2318