Add Synchronous Gauge instrument to the metrics API/SDK

[fgallegosalido]

Before opening a feature request against this repo, consider whether the feature should/could be implemented in the other OpenTelemetry client libraries. If so, please open an issue on opentelemetry-specification first.

Is your feature request related to a problem?
There's plenty of use cases described in the specification issue.

Describe the solution you'd like
I expect to have two new functions in the class Meter called CreateInt64Gauge() and CreateDoubleGauge() along with the template class Gauge. They should behave like the ObservableGauge instrument, but being synchronous instead of asynchronous.

Describe alternatives you've considered
This is described as an experimental feature in the OpenTelemetry specification: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#gauge

Additional context
Add any other context about the feature request here.

[lalitb]

Thanks for creating the issue. Adding for reference the relevant Specs PR -
open-telemetry/opentelemetry-specification#3540

[marcalff]

So:

• this is a recent addition in the spec,
• the meter API is impacted, as it needs a new virtual method,
• this makes it an ABI breaking change for C++.

We really need a plan to handle ABI changes gracefully, see #2222

[marcalff]

Depends on:

• [API] Add InstrumentationScope attributes in MeterProvider::GetMeter() #2224

which will add CI for a WITH_ABI_VERSION_2 build.

[github-actions]

This issue was marked as stale due to lack of activity.