Support multiple-instrument callbacks

[fractalwrench]

Multiple-instrument callbacks

Multiple-instrument callbacks: one callback observing several asynchronous instruments, so a single
expensive read can feed all of them.
Contains the following fields: a callback function, the list of instruments it observes

Spec section: https://opentelemetry.io/docs/specs/otel/metrics/api/#multiple-instrument-callbacks
API types: a Meter.batchCallback(...)-style function (new), ObservableMeasurement

Spec links:

• Multiple-instrument callbacks: https://opentelemetry.io/docs/specs/otel/metrics/api/#multiple-instrument-callbacks
• Asynchronous Instrument API: https://opentelemetry.io/docs/specs/otel/metrics/api/#asynchronous-instrument-api

Implementation checklist:

• Interface declared in api module?
• No-op implementation in noop module?
• Fake implementation in test-fakes module?
• compat adapts Java → Kotlin?
• compat adapts Kotlin → Java?
• implementation records measurements?