No memory leakage in attributes filter (#3695)

The attributes filter collects seen attributes in order to avoid
filtration on the same attribute set. However, the `attribute.Set` is
not comparable type and new allocations of sets with same attributes will be
considered as new sets.

Metrics with a high cardinality of attributes consume a lot of memory
even if we set a filter to reduce that cardinality.

Co-authored-by: Chester Cheung <cheung.zhy.csu@gmail.com>
Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

sdk/metric/internal/filter.go

@@ -15,8 +15,6 @@
 package internal // import "go.opentelemetry.io/otel/sdk/metric/internal"
 
 import (
-	"sync"
-
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/sdk/metric/metricdata"
 )
@@ -44,9 +42,6 @@ func NewFilter[N int64 | float64](agg Aggregator[N], fn attribute.Filter) Aggreg
 type filter[N int64 | float64] struct {
 	filter     attribute.Filter
 	aggregator Aggregator[N]
-
-	sync.Mutex
-	seen map[attribute.Set]attribute.Set
 }
 
 // newFilter returns an filter Aggregator that wraps agg with the attribute
@@ -58,21 +53,13 @@ func newFilter[N int64 | float64](agg Aggregator[N], fn attribute.Filter) *filte
 	return &filter[N]{
 		filter:     fn,
 		aggregator: agg,
-		seen:       make(map[attribute.Set]attribute.Set),
 	}
 }
 
 // Aggregate records the measurement, scoped by attr, and aggregates it
 // into an aggregation.
 func (f *filter[N]) Aggregate(measurement N, attr attribute.Set) {
-	// TODO (#3006): drop stale attributes from seen.
-	f.Lock()
-	defer f.Unlock()
-	fAttr, ok := f.seen[attr]
-	if !ok {
-		fAttr, _ = attr.Filter(f.filter)
-		f.seen[attr] = fAttr
-	}
+	fAttr, _ := attr.Filter(f.filter)
 	f.aggregator.Aggregate(measurement, fAttr)
 }
 
@@ -90,9 +77,6 @@ func (f *filter[N]) Aggregation() metricdata.Aggregation {
 type precomputedFilter[N int64 | float64] struct {
 	filter     attribute.Filter
 	aggregator precomputeAggregator[N]
-
-	sync.Mutex
-	seen map[attribute.Set]attribute.Set
 }
 
 // newPrecomputedFilter returns a precomputedFilter Aggregator that wraps agg
@@ -104,21 +88,13 @@ func newPrecomputedFilter[N int64 | float64](agg precomputeAggregator[N], fn att
 	return &precomputedFilter[N]{
 		filter:     fn,
 		aggregator: agg,
-		seen:       make(map[attribute.Set]attribute.Set),
 	}
 }
 
 // Aggregate records the measurement, scoped by attr, and aggregates it
 // into an aggregation.
 func (f *precomputedFilter[N]) Aggregate(measurement N, attr attribute.Set) {
-	// TODO (#3006): drop stale attributes from seen.
-	f.Lock()
-	defer f.Unlock()
-	fAttr, ok := f.seen[attr]
-	if !ok {
-		fAttr, _ = attr.Filter(f.filter)
-		f.seen[attr] = fAttr
-	}
+	fAttr, _ := attr.Filter(f.filter)
 	if fAttr.Equals(&attr) {
 		// No filtering done.
 		f.aggregator.Aggregate(measurement, fAttr)