[Metrics API/SDK] Support registering multiple async callbacks to one single async instrument.

[lalitb]

Support for specs changes introduced in open-telemetry/opentelemetry-specification#2361 and open-telemetry/opentelemetry-specification#2363

• Add support for registering multiple async callbacks to one single async instrument.
• Every currently registered Callback associated with an instrument MUST
  be evaluated exactly once during collection prior to reading data for
  that instrument.

Depends on
#1334