[deltatocumulative]: limit tracked streams

processor/deltatocumulativeprocessor/README.md

@@ -25,6 +25,8 @@ processors:
     deltatocumulative:
         # how long until a series not receiving new samples is removed
         [ max_stale: <duration> | default = 5m ]
+
+        # upper limit of streams to track. new streams exceeding this limit
+        # will be dropped
+        [ max_streams: <int> | default = off]
 ```
-
-There is no further configuration required. All delta samples are converted to cumulative.

processor/deltatocumulativeprocessor/config.go

@@ -13,12 +13,23 @@ import (
 var _ component.ConfigValidator = (*Config)(nil)
 
 type Config struct {
-	MaxStale time.Duration `json:"max_stale"`
+	MaxStale   time.Duration `json:"max_stale"`
+	MaxStreams int           `json:"max_streams"`
 }
 
 func (c *Config) Validate() error {
 	if c.MaxStale <= 0 {
 		return fmt.Errorf("max_stale must be a positive duration (got %s)", c.MaxStale)
 	}
+	if c.MaxStreams <= 0 {
+		return fmt.Errorf("max_streams must be a positive number (got %d)", c.MaxStreams)
+	}
 	return nil
 }
+
+func createDefaultConfig() component.Config {
+	return &Config{
+		MaxStale:   5 * time.Minute,
+		MaxStreams: 0, // disable. TODO: find good default
+	}
+}

processor/deltatocumulativeprocessor/factory.go

@@ -6,7 +6,6 @@ package deltatocumulativeprocessor // import "github.com/open-telemetry/opentele
 import (
 	"context"
 	"fmt"
-	"time"
 
 	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/consumer"
@@ -23,10 +22,6 @@ func NewFactory() processor.Factory {
 	)
 }
 
-func createDefaultConfig() component.Config {
-	return &Config{MaxStale: 5 * time.Minute}
-}
-
 func createMetricsProcessor(_ context.Context, set processor.CreateSettings, cfg component.Config, next consumer.Metrics) (processor.Metrics, error) {
 	pcfg, ok := cfg.(*Config)
 	if !ok {

processor/deltatocumulativeprocessor/internal/streams/limit.go

@@ -0,0 +1,39 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package streams // import "github.com/open-telemetry/opentelemetry-collector-contrib/processor/deltatocumulativeprocessor/internal/streams"
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/internal/exp/metrics/identity"
+	"github.com/open-telemetry/opentelemetry-collector-contrib/internal/exp/metrics/streams"
+)
+
+func Limit[T any](m Map[T], max int) Map[T] {
+	return LimitMap[T]{Map: m, Max: max}
+}
+
+type LimitMap[T any] struct {
+	Max int
+	streams.Map[T]
+}
+
+func (m LimitMap[T]) Store(id identity.Stream, v T) error {
+	if m.Map.Len() >= m.Max {
+		return ErrLimit(m.Max)
+	}
+	return m.Map.Store(id, v)
+}
+
+type ErrLimit int
+
+func (e ErrLimit) Error() string {
+	return fmt.Sprintf("stream limit of %d reached", e)
+}
+
+func AtLimit(err error) bool {
+	var errLimit ErrLimit
+	return errors.As(err, &errLimit)
+}

processor/deltatocumulativeprocessor/internal/streams/limit_test.go

@@ -0,0 +1,51 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package streams_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/internal/exp/metrics/identity"
+	exp "github.com/open-telemetry/opentelemetry-collector-contrib/internal/exp/metrics/streams"
+	"github.com/open-telemetry/opentelemetry-collector-contrib/processor/deltatocumulativeprocessor/internal/data"
+	"github.com/open-telemetry/opentelemetry-collector-contrib/processor/deltatocumulativeprocessor/internal/streams"
+	"github.com/open-telemetry/opentelemetry-collector-contrib/processor/deltatocumulativeprocessor/internal/testdata/random"
+)
+
+func TestLimit(t *testing.T) {
+	sum := random.Sum()
+
+	items := make(exp.HashMap[data.Number])
+	lim := streams.Limit(items, 10)
+
+	ids := make([]identity.Stream, 10)
+
+	// write until limit must work
+	for i := 0; i < 10; i++ {
+		id, dp := sum.Stream()
+		ids[i] = id
+		err := lim.Store(id, dp)
+		require.NoError(t, err)
+	}
+
+	// one over limit must be rejected
+	{
+		id, dp := sum.Stream()
+		err := lim.Store(id, dp)
+		want := streams.ErrLimit(10)
+		require.ErrorAs(t, err, &want)
+		require.True(t, streams.AtLimit(err))
+	}
+
+	// after removing one, must be accepted again
+	{
+		lim.Delete(ids[0])
+
+		id, dp := sum.Stream()
+		err := lim.Store(id, dp)
+		require.NoError(t, err)
+	}
+}

processor/deltatocumulativeprocessor/processor.go

@@ -53,6 +53,9 @@ func newProcessor(cfg *Config, log *zap.Logger, next consumer.Metrics) *Processo
 		proc.exp = &exp
 		dps = &exp
 	}
+	if cfg.MaxStreams > 0 {
+		dps = streams.Limit(dps, cfg.MaxStreams)
+	}
 
 	proc.aggr = streams.IntoAggregator(dps)
 	return &proc