Metrics: add asynchronous instruments

[inge4pres]

Describe the current state

The OTel Metrics API defines synchronous and asynchronous instruments.

So far, only synchronous instruments are implemented.

Describe the desired state

We implement asynchronous instruments.

[inge4pres]

To do:

• Async Gauge
• Async Counter

The Async UpDownCounter is already done when developing the Counter, because of how we use generic types for instruments (e.g. an Counter(i32) is an UpDownCounter(i32) in instrumentData).

For each instrument we should develop the recording of data with a user-provided callback, as described in the API docs.