Make it possible to extract fields from the context in the slog adapter

exp/zapslog/handler.go

@@ -32,13 +32,17 @@ import (
 	"go.uber.org/zap/zapcore"
 )
 
+// ContextFieldExtractor can be used to extract a field from a context.
+type ContextFieldExtractor func(ctx context.Context) []zapcore.Field
+
 // Handler implements the slog.Handler by writing to a zap Core.
 type Handler struct {
-	core       zapcore.Core
-	name       string // logger name
-	addCaller  bool
-	addStackAt slog.Level
-	callerSkip int
+	core                   zapcore.Core
+	name                   string // logger name
+	addCaller              bool
+	addStackAt             slog.Level
+	callerSkip             int
+	contextFieldExtractors []ContextFieldExtractor
 }
 
 // NewHandler builds a [Handler] that writes to the supplied [zapcore.Core]
@@ -157,7 +161,14 @@ func (h *Handler) Handle(ctx context.Context, record slog.Record) error {
 		ce.Stack = stacktrace.Take(3 + h.callerSkip)
 	}
 
-	fields := make([]zapcore.Field, 0, record.NumAttrs())
+	var contextFields []zapcore.Field
+	for _, extractor := range h.contextFieldExtractors {
+		contextFields = append(contextFields, extractor(ctx)...)
+	}
+
+	fields := make([]zapcore.Field, 0, record.NumAttrs()+len(contextFields))
+	fields = append(fields, contextFields...)
+
 	record.Attrs(func(attr slog.Attr) bool {
 		fields = append(fields, convertAttrToField(attr))
 		return true

exp/zapslog/handler_test.go

@@ -23,16 +23,20 @@
 package zapslog
 
 import (
+	"context"
 	"log/slog"
 	"testing"
 	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"go.uber.org/zap"
 	"go.uber.org/zap/zapcore"
 	"go.uber.org/zap/zaptest/observer"
 )
 
+type testContextKey string
+
 func TestAddCaller(t *testing.T) {
 	t.Parallel()
 
@@ -189,3 +193,19 @@ func TestAttrKinds(t *testing.T) {
 		},
 		entry.ContextMap())
 }
+
+func TestContextFieldExtractor(t *testing.T) {
+	key := testContextKey("testkey")
+	fac, logs := observer.New(zapcore.DebugLevel)
+	ctx := context.WithValue(context.Background(), key, "testvalue")
+
+	sl := slog.New(NewHandler(fac, WithContextFieldExtractors(func(ctx context.Context) []zapcore.Field {
+		v := ctx.Value(key).(string)
+		return []zapcore.Field{zap.String("testkey", v)}
+	})))
+	sl.InfoContext(ctx, "msg")
+	lines := logs.TakeAll()
+
+	require.Len(t, lines, 1)
+	require.Equal(t, "testvalue", lines[0].ContextMap()["testkey"])
+}

exp/zapslog/options.go

@@ -70,3 +70,10 @@
 		log.addStackAt = lvl
 	})
 }
+
+// .WithContextFieldExtractors configures the Logger to extract fields from the context.
+func WithContextFieldExtractors(extractors ...ContextFieldExtractor) Option {
+	return optionFunc(func(log *Handler) {
+		log.contextFieldExtractors = append(log.contextFieldExtractors, extractors...)
+	})
+}