[processor/deltatocumulative] limit tracked streams (#31488)

**Description:** Adds a configurable upper limit to the number of
tracked streams. This allows to introduce a upper bound to the memory
usage.

**Testing:** Test case was added

**Documentation:** README was updated

.chloggen/deltatocumulative-limits.yaml

@@ -0,0 +1,30 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: "enhancement"
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: "deltatocumulativeprocessor"
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: introduce configurable stream limit
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [31488]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: |
+  Adds `max_streams` option that allows to set upper bound (default = unlimited)
+  to the number of tracked streams. Any additional streams exceeding the limit
+  are dropped.
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

internal/exp/metrics/staleness/staleness.go

@@ -13,6 +13,11 @@ import (
 // We override how Now() is returned, so we can have deterministic tests
 var NowFunc = time.Now
 
+var (
+	_ streams.Map[any] = (*Staleness[any])(nil)
+	_ streams.Evictor  = (*Staleness[any])(nil)
+)
+
 // Staleness a a wrapper over a map that adds an additional "staleness" value to each entry. Users can
 // call ExpireOldEntries() to automatically remove all entries from the map whole staleness value is
 // older than the `max`
@@ -82,3 +87,9 @@ func (s *Staleness[T]) Next() time.Time {
 	_, ts := s.pq.Peek()
 	return ts
 }
+
+func (s *Staleness[T]) Evict() identity.Stream {
+	id, _ := s.pq.Pop()
+	s.items.Delete(id)
+	return id
+}

internal/exp/metrics/streams/streams.go

@@ -50,3 +50,9 @@ func (m HashMap[T]) Items() func(yield func(identity.Stream, T) bool) bool {
 func (m HashMap[T]) Len() int {
 	return len((map[identity.Stream]T)(m))
 }
+
+// Evictors remove the "least important" stream based on some strategy such as
+// the oldest, least active, etc.
+type Evictor interface {
+	Evict() identity.Stream
+}

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 = 0 (off) ]
 ```
-
-There is no further configuration required. All delta samples are converted to cumulative.

processor/deltatocumulativeprocessor/config.go

@@ -13,12 +13,26 @@ 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,
+
+		// disable. TODO: find good default
+		// https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/31603
+		MaxStreams: 0,
+	}
+}

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 +1,63 @@
+// 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) LimitMap[T] {
+	return LimitMap[T]{Map: m, Max: max}
+}
+
+type LimitMap[T any] struct {
+	Max int
+
+	Evictor streams.Evictor
+	streams.Map[T]
+}
+
+func (m LimitMap[T]) Store(id identity.Stream, v T) error {
+	if m.Map.Len() < m.Max {
+		return m.Map.Store(id, v)
+	}
+
+	errl := ErrLimit(m.Max)
+	if m.Evictor != nil {
+		gone := m.Evictor.Evict()
+		if err := m.Map.Store(id, v); err != nil {
+			return err
+		}
+		return ErrEvicted{ErrLimit: errl, id: gone}
+	}
+	return errl
+}
+
+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)
+}
+
+type ErrEvicted struct {
+	ErrLimit
+	id Ident
+}
+
+func (e ErrEvicted) Error() string {
+	return fmt.Sprintf("%s. evicted stream %s", e.ErrLimit, e.id)
+}
+
+func (e ErrEvicted) Unwrap() error {
+	return e.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/internal/streams/streams.go

@@ -33,3 +33,5 @@ func (a MapAggr[D]) Aggregate(id Ident, dp D) (D, error) {
 	v, _ := a.Map.Load(id)
 	return v, err
 }
+
+type Evictor = streams.Evictor

processor/deltatocumulativeprocessor/processor.go

@@ -55,6 +55,13 @@ func newProcessor(cfg *Config, log *zap.Logger, next consumer.Metrics) *Processo
 		proc.stale = stale
 		dps = stale
 	}
+	if cfg.MaxStreams > 0 {
+		lim := streams.Limit(dps, cfg.MaxStreams)
+		if proc.exp != nil {
+			lim.Evictor = proc.exp
+		}
+		dps = lim
+	}
 
 	proc.aggr = streams.IntoAggregator(dps)
 	return &proc