Asynchronous Instrument API does not follow OTel Specification recommendation on naming

[MrAlias]

The OpenTelemetry specification recommends naming asynchronous instrument with certain names:

Asynchronous Counter creation

It is highly recommended that implementations use the name ObservableCounter (or any language idiomatic variation, e.g. observable_counter) unless there is a strong reason not to do so.

Asynchronous Gauge creation

It is highly recommended that implementations use the name ObservableGauge (or any language idiomatic variation, e.g. observable_gauge) unless there is a strong reason not to do so.

Asynchronous UpDownCounter creation

It is highly recommended that implementations use the name ObservableUpDownCounter (or any language idiomatic variation, e.g. observable_updowncounter) unless there is a strong reason not to do so.

The opentelemetry-go project does not currently adhere to this naming strategy. I do not see a strong reason to not do so either.

[MrAlias]

I think this may be solved by #3530.

I'm not sure if the specification wants the type names to also be Observable* or just the methods. The quoted sections are for the creation methods of each instrument, which makes me think it is recommending the method name only. Not 100% sure though.