*Impl type names are confusing

[rakyll]

Per spec, https://github.com/open-telemetry/opentelemetry-specification/tree/master/specification/metrics, there are various metrics interfaces that sit in the boundary of the SDK and the API. These types are named as "*Impl" which resembles "implementation" types from other languages. Is there a better way to rename them to avoid the confusion that they are concrete types.

For example, MeterImpl sounds like a concrete type for Meter even though it's not. In Go, *Impl is not a naming convention for concrete types either. So the API surface looks unidiomatic and harder to navigate from the users perspective.

[MrAlias]

Resolved by #2587