ValueObserver should use LastValue aggregation by default

[nilebox]

What are you trying to achieve?
According to the comment #636 (comment), the agreement is to use LastValue aggregation for ValueObserver.
The specification still states that ValueObserver should have MinMaxSumCount aggregation by default: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/metrics/api.md#aggregations

The non-additive instruments (ValueRecorder, ValueObserver) use a MinMaxSumCount aggregation, by default.

What did you expect to see?

Additional context.

Code change in Go SDK switching to LastValue: open-telemetry/opentelemetry-go#1165, needs to be made in other languages as well, but first we need to update the spec.

[nilebox]

Closing as duplicate of #834