Multiple-instrument Callbacks must only be associated with instruments from the same Meter

[MrAlias]

The metric specification states:

Multiple-instrument Callbacks MUST be associated at the time of registration with a declared set of asynchronous instruments from the same Meter instance.

Currently, our SDK implementation does not conform to this. In fact we test that it violates this requirement currently:

opentelemetry-go/sdk/metric/meter_test.go

Lines 514 to 521 in 96d5de1

	m2 := mp.Meter("scope2")
	ctr2, err := m2.Int64ObservableCounter("ctr2")
	assert.NoError(t, err)
	_, err = m1.RegisterCallback([]instrument.Asynchronous{ctr2}, func(ctx context.Context) error {
	ctr2.Observe(ctx, 7)
	return nil
	})
	assert.NoError(t, err)

[MrAlias]

I have a proposal to solve #3519 that I am working on. It should also resolve this issue.