Compatible aggregators should not error instrument creation

[MrAlias]

The OTel spec says:

• For each View, if the Instrument could match the instrument selection criteria:
  • Try to apply the View configuration. If applying the View results in conflicting metric identities the implementation SHOULD apply the View and emit a warning. If it is not possible to apply the View without producing semantic errors (e.g. the View sets an asynchronous instrument to use the Explicit bucket histogram aggregation) the implementation SHOULD emit a warning and proceed as if the View did not exist.

If a view translates an instrument into the exact same as another instrument the same aggregators from the already registered instrument should be updated and pulled from.

Currently, we do not check compatibility:

opentelemetry-go/sdk/metric/pipeline.go

Lines 107 to 109 in aca054b

	if _, ok := p.aggregations[scope][inst]; ok {
	return fmt.Errorf("%w: name %s, scope: %s", errAlreadyRegistered, name, scope)
	}

Instead, we just return an error saying the instrument was already registered.

To make matters worse, with the error we return an internal.Aggregator that will never be pulled from:

opentelemetry-go/sdk/metric/pipeline.go

Lines 229 to 238 in aca054b

	aggs = append(aggs, agg)
	seen[id] = struct{}{}
	err = i.pipeline.addAggregator(inst.Scope, inst.Name, inst.Description, instUnit, agg)
	if err != nil {
	errs.append(err)
	}
	}
	// TODO(#3224): handle when no views match. Default should be reader
	// aggregation returned.
	return aggs, errs.errorOrNil()

Environment

• opentelemetry-go version: aca054b

Steps To Reproduce

Change this line to check for NoError

opentelemetry-go/sdk/metric/pipeline_registry_test.go

Line 415 in aca054b

assert.Error(t, err)

Run the tests

$ go test .
--- FAIL: TestPipelineRegistryCreateAggregatorsDuplicateErrors (0.00s)
    pipeline_registry_test.go:415:
        	Error Trace:	pipeline_registry_test.go:415
        	Error:      	Received unexpected error:
        	            	%!w(<nil>): could not create all aggregators: instrument already registered: name foo, scope: {  }
        	Test:       	TestPipelineRegistryCreateAggregatorsDuplicateErrors
FAIL
FAIL	go.opentelemetry.io/otel/sdk/metric	0.010s
FAIL

Expected behavior

Test should pass with that modification and the returned internal.Aggregator should update the existing registered on.e