[api-metrics]: Create or verify compliance of Async Counter

[dyladan]

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#asynchronous-counter

• Creation method
  • name (required)
  • callback (required)
  • unit (optional string)
  • description (optional string)
• Interface
  • No methods
  • callback
    • observer argument
      • observe method
• Noop implementation

[legendecas]

When the batch observer is available, the callback shall be optional if I understand correctly. But as the batch observer is not included in the feature freeze API spec, I have no idea whether or not we should change the API definition for now.

[dyladan]

As of now the status seems the same as in #2548:

• feat(api-metrics): remove bind/unbind and bound instruments #2559 removes bound version
• Attributes should be made optional
• Optional context should be added

[dyladan]

@legendecas batch observer was removed from the spec along with bind in order to simplify the spec for v1 so the release can go as smoothly as possible. After v1 it will be added back in afaik. For now we should remove it since we can't know what the final implementation will look like.