Use default view if inst matches no other

Fix #3224

sdk/metric/pipeline.go

@@ -188,31 +188,23 @@ func newInserter[N int64 | float64](p *pipeline) *inserter[N] {
 // Instrument inserts instrument inst with instUnit returning the Aggregators
 // that need to be updated with measurments for that instrument.
 func (i *inserter[N]) Instrument(inst view.Instrument, instUnit unit.Unit) ([]internal.Aggregator[N], error) {
+	var matched bool
 	seen := map[instrumentID]struct{}{}
 	var aggs []internal.Aggregator[N]
 	errs := &multierror{wrapped: errCreatingAggregators}
 	for _, v := range i.pipeline.views {
 		inst, match := v.TransformInstrument(inst)
+		if !match {
+			continue
+		}
+		matched = true
 
 		id := instrumentID{
 			scope:       inst.Scope,
 			name:        inst.Name,
 			description: inst.Description,
 		}
-
-		if _, ok := seen[id]; ok || !match {
-			continue
-		}
-
-		if inst.Aggregation == nil {
-			inst.Aggregation = i.pipeline.reader.aggregation(inst.Kind)
-		} else if _, ok := inst.Aggregation.(aggregation.Default); ok {
-			inst.Aggregation = i.pipeline.reader.aggregation(inst.Kind)
-		}
-
-		if err := isAggregatorCompatible(inst.Kind, inst.Aggregation); err != nil {
-			err = fmt.Errorf("creating aggregator with instrumentKind: %d, aggregation %v: %w", inst.Kind, inst.Aggregation, err)
-			errs.append(err)
+		if _, ok := seen[id]; ok {
 			continue
 		}
 
@@ -233,16 +225,40 @@ func (i *inserter[N]) Instrument(inst view.Instrument, instUnit unit.Unit) ([]in
 			errs.append(err)
 		}
 	}
-	// TODO(#3224): handle when no views match. Default should be reader
-	// aggregation returned.
+
+	if !matched { // Apply implicit default view if no explicit matched.
+		a, err := i.aggregator(inst)
+		if err != nil {
+			errs.append(err)
+		}
+		if a != nil {
+			aggs = append(aggs, a)
+			err = i.pipeline.addAggregator(inst.Scope, inst.Name, inst.Description, instUnit, a)
+			if err != nil {
+				errs.append(err)
+			}
+		}
+	}
+
 	return aggs, errs.errorOrNil()
 }
 
 // aggregator returns the Aggregator for an instrument configuration. If the
 // instrument defines an unknown aggregation, an error is returned.
 func (i *inserter[N]) aggregator(inst view.Instrument) (internal.Aggregator[N], error) {
-	// TODO (#3011): If filtering is done by the Aggregator it should be passed
-	// here.
+	switch inst.Aggregation.(type) {
+	case nil, aggregation.Default:
+		// Undefined, nil, means to use the default from the reader.
+		inst.Aggregation = i.pipeline.reader.aggregation(inst.Kind)
+	}
+
+	if err := isAggregatorCompatible(inst.Kind, inst.Aggregation); err != nil {
+		return nil, fmt.Errorf(
+			"creating aggregator with instrumentKind: %d, aggregation %v: %w",
+			inst.Kind, inst.Aggregation, err,
+		)
+	}
+
 	var (
 		temporality = i.pipeline.reader.temporality(inst.Kind)
 		monotonic   bool