sdk/metric: NewManualReader and NewPeriodicReader should return structs

[pellared]

From https://github.com/open-telemetry/opentelemetry-specification/blob/v1.21.0/specification/metrics/sdk.md#compatibility-requirements:

All the metrics components SHOULD allow new methods to be added to existing components without introducing breaking changes.

Adding new methods to struct in Go is not a breaking change. Most of the Go components are structs. However some methods also accept interfaces.

Adding a new method to a interface is a breaking change.

NewManualReader and NewPeriodicReader are components returned as interfaces. It is would not possible to add a new methods (e.g. Pause and Continue) to periodic reader API without creating a new method like NewPeriodicReaderBis that would return a struct (or other interface). I think it would be more flexible if both methods return structs instead of Reader interface.

Originally posted by @pellared in #3671 (comment)