Move synchronous gauge to stability

[nathanieltalbot]

Is your feature request related to a problem?
There is an incoming metrics API change in the opentelemetry specification to support a synchronous gauge instrument: open-telemetry/opentelemetry-specification#3540.

This is spawned off of a long-running feature request in the specification for this instrument and a settable UpDownCounter: open-telemetry/opentelemetry-specification#2318.

This should be implemented into the Python SDK once this specification change is merged.

Describe the solution you'd like
Implementation of the Synchronous Gauge instrument in the Python SDK.

Describe alternatives you've considered
If the OpenTelemetry Specification decides not to support this then this change is not needed.

[freemansoft]

Is there an expected release date that contains #3462

[freemansoft]

Looks like this is available in https://github.com/open-telemetry/opentelemetry-python/releases/tag/v1.23.0. This open issue can probably be closed.

[lzchen]

Synchronous gauge is now marked as stable! open-telemetry/opentelemetry-specification#4019. This will now be a tracking issue to move this component to stability.