Support .NET 9 Synchronous Gauge

[tommaybe]

Feature Request

It would be great for synchronous gauge support to be added, now that it has been added to Experimental spec.

Is your feature request related to a problem?

Use cases are described here: open-telemetry/opentelemetry-specification#2318.

Describe the solution you'd like

I would expect to have a method in the class Meter called CreateObservableGauge() along with the class Gauge. They should behave like the ObservableGauge instrument, but being synchronous instead of asynchronous.

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

Describe alternatives you've considered

Both UpDownCounter and ObservableGauge do not work for my usecases.

Additional context

[cijothomas]

Added the runtime-change needed label, as the instruments must be added in .NET Runtime and then OTel .NET (this repo) need to add support. This will come in .NET 9 timeframe only as .NET 8 train already left. Will open tracking issue in runtime repo, and link to this.

[cijothomas]

.NET Runtime proposal is here : dotnet/runtime#92625 (comment)