Document the callback registered with a Meter according to the specification

[MrAlias]

The OpenTelemetry specification states:

Callback functions MUST be documented as follows for the end user:

• Callback functions SHOULD be reentrant safe. The SDK expects to evaluate callbacks for each MetricReader independently.
• Callback functions SHOULD NOT take an indefinite amount of time.
• Callback functions SHOULD NOT make duplicate observations (more than one Measurement with the same attributes) across all registered callbacks.

Currently the callback passed to a Meter's RegisterCallback method is not documented as such:

opentelemetry-go/metric/meter.go

Lines 98 to 109 in efd8a7d

	// RegisterCallback registers f to be called during the collection of a
	// measurement cycle.
	//
	// If Unregister of the returned Registration is called, f needs to be
	// unregistered and not called during collection.
	//
	// The instruments f is registered with are the only instruments that f may
	// observe values for.
	//
	// If no instruments are passed, f should not be registered nor called
	// during collection.
	RegisterCallback(instruments []instrument.Asynchronous, f func(context.Context)) (Registration, error)

Proposal

Define a Callback type in that file that takes the place of func(context.Context). Document that type with pertinent information, including the OTel required information. For example,

type Meter interface {
	// ...
	RegisterCallback(instruments []instrument.Asynchronous, f Callback) (Registration, error)
}

// Callback is a function registered with a Meter that makes observations for
// the set of instruments it is registered with.
//
// The function needs to complete in a finite amount of time and the deadline
// of the passed context is expected to be honored.
//
// The function needs to make unique observations across all registered
// callbacks. It should not report duplicate measurements for an instruments
// with the same attributes as another Callback.
//
// The function needs to be concurrent safe.
type Callback func(context.Context)