[sdk-metrics] Do not report metric data for previosuly observed attribute sets that were not observed during the current cycle

[pichlermarc]

Description

Currently, we do keep reporting even if the callback has been registered and no observations do take place (see #2997 (comment))

However, the spec states:

The implementation SHOULD NOT produce aggregated metric data for a
previously-observed attribute set which is not observed during a successful
callback.

Describe the solution you'd like

Implement the specification.

Additional resources

• Ability to remove/flush unused attributes in metric instruments #2997
• Attribute sets not observed during async callbacks are not exported opentelemetry-specification#3242