baggagetrace: Add baggage key predicate (open-telemetry#5619)

## Which problem is this PR solving?
- Closes open-telemetry#5437 

## Short description of the changes
- Add `BaggageKeyPredicate` type and `AllowAllBaggageKeys`
implementation that allows all keys
- Add baggage key predicate as constructor parameter to processor
- Add unit tests to verify behaviour for prefix and regex
- Update README with examples of setting a filter

---------

Co-authored-by: Alex Boten <223565+codeboten@users.noreply.github.com>

CHANGELOG.md

@@ -71,6 +71,8 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
   This parameter is used as a replacement of `WithInstrumentationScope` to specify the name of the logger backing the returned `Handler`. (#5588)
 - Upgrade all dependencies of `go.opentelemetry.io/otel/semconv/v1.24.0` to `go.opentelemetry.io/otel/semconv/v1.25.0`. (#5605)
 
+- Update the span processor in `go.opentelemetry.io/contrib/processors/baggage/baggagetrace` to require a baggage key predicate. (#5619)
+
 ### Removed
 
 - The `WithInstrumentationScope` option function in `go.opentelemetry.io/contrib/bridges/otelslog` is removed.

processors/baggage/baggagetrace/doc.go

@@ -1,5 +1,27 @@
 // Copyright The OpenTelemetry Authors
 // SPDX-License-Identifier: Apache-2.0
 
-// Package baggagetrace implements a baggage span processor.
+// Package baggagetrace is an OpenTelemetry [Span Processor] that reads key/values
+// stored in [Baggage] in the starting span's parent context and adds them as
+// attributes to the span.
+//
+// Keys and values added to Baggage will appear on all subsequent child spans for
+// a trace within this service and will be propagated to external services via
+// propagation headers.
+// If the external services also have a Baggage span processor, the keys and
+// values will appear in those child spans as well.
+//
+// Do not put sensitive information in Baggage.
+//
+// # Usage
+//
+// Add the span processor when configuring the tracer provider.
+//
+// The convenience function [AllowAllBaggageKeys] is provided to
+// allow all baggage keys to be copied to the span. Alternatively, you can
+// provide a custom baggage key predicate to select which baggage keys you want
+// to copy.
+//
+// [Span Processor]: https://opentelemetry.io/docs/specs/otel/trace/sdk/#span-processor
+// [Baggage]: https://opentelemetry.io/docs/specs/otel/api/baggage
 package baggagetrace // import "go.opentelemetry.io/contrib/processors/baggage/baggagetrace"

processors/baggage/baggagetrace/example_test.go

@@ -0,0 +1,43 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package baggagetrace_test
+
+import (
+	"regexp"
+	"strings"
+
+	"go.opentelemetry.io/contrib/processors/baggage/baggagetrace"
+	"go.opentelemetry.io/otel/sdk/trace"
+)
+
+func ExampleNew_allKeys() {
+	trace.NewTracerProvider(
+		trace.WithSpanProcessor(baggagetrace.New(baggagetrace.AllowAllBaggageKeys)),
+	)
+}
+
+func ExampleNew_keysWithPrefix() {
+	trace.NewTracerProvider(
+		trace.WithSpanProcessor(
+			baggagetrace.New(
+				func(baggageKey string) bool {
+					return strings.HasPrefix(baggageKey, "my-key")
+				},
+			),
+		),
+	)
+}
+
+func ExampleNew_keysMatchingRegex() {
+	expr := regexp.MustCompile(`^key.+`)
+	trace.NewTracerProvider(
+		trace.WithSpanProcessor(
+			baggagetrace.New(
+				func(baggageKey string) bool {
+					return expr.MatchString(baggageKey)
+				},
+			),
+		),
+	)
+}

processors/baggage/baggagetrace/processor.go

@@ -11,23 +11,36 @@ import (
 	"go.opentelemetry.io/otel/sdk/trace"
 )
 
+// BaggageKeyPredicate is a function that returns true if the baggage key should be added to the span.
+type BaggageKeyPredicate func(baggageKey string) bool
+
+// AllowAllBaggageKeys is a BaggageKeyPredicate that allows all baggage keys.
+var AllowAllBaggageKeys = func(string) bool { return true }
+
 // SpanProcessor is a processing pipeline for spans in the trace signal.
-type SpanProcessor struct{}
+type SpanProcessor struct {
+	baggageKeyPredicate BaggageKeyPredicate
+}
 
 var _ trace.SpanProcessor = (*SpanProcessor)(nil)
 
 // New returns a new SpanProcessor.
 //
 // The Baggage span processor duplicates onto a span the attributes found
 // in Baggage in the parent context at the moment the span is started.
-func New() trace.SpanProcessor {
-	return &SpanProcessor{}
+// The predicate function is used to filter which baggage keys are added to the span.
+func New(baggageKeyPredicate BaggageKeyPredicate) trace.SpanProcessor {
+	return &SpanProcessor{
+		baggageKeyPredicate: baggageKeyPredicate,
+	}
 }
 
 // OnStart is called when a span is started and adds span attributes for baggage contents.
 func (processor SpanProcessor) OnStart(ctx context.Context, span trace.ReadWriteSpan) {
 	for _, entry := range otelbaggage.FromContext(ctx).Members() {
-		span.SetAttributes(attribute.String(entry.Key(), entry.Value()))
+		if processor.baggageKeyPredicate(entry.Key()) {
+			span.SetAttributes(attribute.String(entry.Key(), entry.Value()))
+		}
 	}
 }
 

processors/baggage/baggagetrace/processor_test.go

@@ -5,9 +5,10 @@ package baggagetrace
 
 import (
 	"context"
+	"regexp"
+	"strings"
 	"testing"
 
-	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
 	"go.opentelemetry.io/otel/attribute"
@@ -33,19 +34,15 @@ func NewTestExporter() *testExporter {
 	return &testExporter{}
 }
 
-func TestSpanProcessorAppendsBaggageAttributes(t *testing.T) {
-	suitcase, err := otelbaggage.New()
-	require.NoError(t, err)
-	packingCube, err := otelbaggage.NewMemberRaw("baggage.test", "baggage value")
-	require.NoError(t, err)
-	suitcase, err = suitcase.SetMember(packingCube)
-	require.NoError(t, err)
-	ctx := otelbaggage.ContextWithBaggage(context.Background(), suitcase)
+func TestSpanProcessorAppendsAllBaggageAttributes(t *testing.T) {
+	baggage, _ := otelbaggage.New()
+	baggage = addEntryToBaggage(t, baggage, "baggage.test", "baggage value")
+	ctx := otelbaggage.ContextWithBaggage(context.Background(), baggage)
 
 	// create trace provider with baggage processor and test exporter
 	exporter := NewTestExporter()
 	tp := trace.NewTracerProvider(
-		trace.WithSpanProcessor(New()),
+		trace.WithSpanProcessor(New(AllowAllBaggageKeys)),
 		trace.WithSpanProcessor(trace.NewSimpleSpanProcessor(exporter)),
 	)
 
@@ -54,9 +51,103 @@ func TestSpanProcessorAppendsBaggageAttributes(t *testing.T) {
 	_, span := tracer.Start(ctx, "test")
 	span.End()
 
-	assert.Len(t, exporter.spans, 1)
-	assert.Len(t, exporter.spans[0].Attributes(), 1)
+	require.Len(t, exporter.spans, 1)
+	require.Len(t, exporter.spans[0].Attributes(), 1)
 
 	want := []attribute.KeyValue{attribute.String("baggage.test", "baggage value")}
-	assert.Equal(t, want, exporter.spans[0].Attributes())
+	require.Equal(t, want, exporter.spans[0].Attributes())
+}
+
+func TestSpanProcessorAppendsBaggageAttributesWithHaPrefixPredicate(t *testing.T) {
+	baggage, _ := otelbaggage.New()
+	baggage = addEntryToBaggage(t, baggage, "baggage.test", "baggage value")
+	ctx := otelbaggage.ContextWithBaggage(context.Background(), baggage)
+
+	baggageKeyPredicate := func(key string) bool {
+		return strings.HasPrefix(key, "baggage.")
+	}
+
+	// create trace provider with baggage processor and test exporter
+	exporter := NewTestExporter()
+	tp := trace.NewTracerProvider(
+		trace.WithSpanProcessor(New(baggageKeyPredicate)),
+		trace.WithSpanProcessor(trace.NewSimpleSpanProcessor(exporter)),
+	)
+
+	// create tracer and start/end span
+	tracer := tp.Tracer("test")
+	_, span := tracer.Start(ctx, "test")
+	span.End()
+
+	require.Len(t, exporter.spans, 1)
+	require.Len(t, exporter.spans[0].Attributes(), 1)
+
+	want := []attribute.KeyValue{attribute.String("baggage.test", "baggage value")}
+	require.Equal(t, want, exporter.spans[0].Attributes())
+}
+
+func TestSpanProcessorAppendsBaggageAttributesWithRegexPredicate(t *testing.T) {
+	baggage, _ := otelbaggage.New()
+	baggage = addEntryToBaggage(t, baggage, "baggage.test", "baggage value")
+	ctx := otelbaggage.ContextWithBaggage(context.Background(), baggage)
+
+	expr := regexp.MustCompile(`^baggage\..*`)
+	baggageKeyPredicate := func(key string) bool {
+		return expr.MatchString(key)
+	}
+
+	// create trace provider with baggage processor and test exporter
+	exporter := NewTestExporter()
+	tp := trace.NewTracerProvider(
+		trace.WithSpanProcessor(New(baggageKeyPredicate)),
+		trace.WithSpanProcessor(trace.NewSimpleSpanProcessor(exporter)),
+	)
+
+	// create tracer and start/end span
+	tracer := tp.Tracer("test")
+	_, span := tracer.Start(ctx, "test")
+	span.End()
+
+	require.Len(t, exporter.spans, 1)
+	require.Len(t, exporter.spans[0].Attributes(), 1)
+
+	want := []attribute.KeyValue{attribute.String("baggage.test", "baggage value")}
+	require.Equal(t, want, exporter.spans[0].Attributes())
+}
+
+func TestOnlyAddsBaggageEntriesThatMatchPredicate(t *testing.T) {
+	baggage, _ := otelbaggage.New()
+	baggage = addEntryToBaggage(t, baggage, "baggage.test", "baggage value")
+	baggage = addEntryToBaggage(t, baggage, "foo", "bar")
+	ctx := otelbaggage.ContextWithBaggage(context.Background(), baggage)
+
+	baggageKeyPredicate := func(key string) bool {
+		return key == "baggage.test"
+	}
+
+	// create trace provider with baggage processor and test exporter
+	exporter := NewTestExporter()
+	tp := trace.NewTracerProvider(
+		trace.WithSpanProcessor(New(baggageKeyPredicate)),
+		trace.WithSpanProcessor(trace.NewSimpleSpanProcessor(exporter)),
+	)
+
+	// create tracer and start/end span
+	tracer := tp.Tracer("test")
+	_, span := tracer.Start(ctx, "test")
+	span.End()
+
+	require.Len(t, exporter.spans, 1)
+	require.Len(t, exporter.spans[0].Attributes(), 1)
+
+	want := attribute.String("baggage.test", "baggage value")
+	require.Equal(t, want, exporter.spans[0].Attributes()[0])
+}
+
+func addEntryToBaggage(t *testing.T, baggage otelbaggage.Baggage, key, value string) otelbaggage.Baggage {
+	member, err := otelbaggage.NewMemberRaw(key, value)
+	require.NoError(t, err)
+	baggage, err = baggage.SetMember(member)
+	require.NoError(t, err)
+	return baggage
 }