Multiple-instrument callbacks for observables

[mortenpi]

This is a bit similar to #85, but distinct in that the idea is to set multiple different instruments in one callback call (potentially different types of instruments, so attributes are not a suitable workaround either I think).

The spec has a suggestion for an API: https://opentelemetry.io/docs/specs/otel/metrics/api/#multiple-instrument-callbacks

Is there some way to achieve this right now? If not (which I think is the case), is this something that should be supported? Any thoughts on the API?

[findmyway]

Should be easy to support.

（I had thought about this while reading data from NVSwitch;）