[Metrics API/SDK] Change Meter API/SDK to return nostd::unique_ptr for Sync Instruments

[lalitb]

The Sync Instruments created through Meter API are not used internally within SDK. So it would be good to return nostd::unique_ptr for them. Changes are required in API and SDK for

• Counter ( UINT64 and Double)
• Histogram ( UINT64 and Double)
• UpDownCounter ( INT64 and Double)

The Async Instruments can continue to return nostd::shared_ptr.
As part of this or separate PR, we can also maintain the shared_ptr of Async Instrument (instead of current raw pointer) in Observable Registry

opentelemetry-cpp/sdk/include/opentelemetry/sdk/metrics/state/observable_registry.h

Line 24 in ac105bf

opentelemetry::metrics::ObservableInstrument *instrument;

[esigo]

It's not possible to maintain a shared_ptr of Async Instrument

opentelemetry-cpp/sdk/include/opentelemetry/sdk/metrics/state/observable_registry.h

Line 24 in ac105bf

opentelemetry::metrics::ObservableInstrument *instrument;

as we need to enable shared from this, which doesn't work with nostd::shared_ptr.

[lalitb]

as we need to enable shared from this, which doesn't work with nostd::shared_ptr.

Thanks for trying. We can handle this as separate PR. As this is internal SDK change, should be fine to handle post GA. Please raise a issue for this.