Add support for Unwrap []error (#1400)

error.go

@@ -469,6 +469,10 @@ func (b *exceptionDataBuilder) init(e *exceptionData, err error) bool {
 		if cause := err.Unwrap(); cause != nil {
 			e.ErrorDetails.Cause = append(e.ErrorDetails.Cause, cause)
 		}
+	case interface{ Unwrap() []error }:
+		if causes := err.Unwrap(); causes != nil {
+			e.ErrorDetails.Cause = append(e.ErrorDetails.Cause, causes...)
+		}
 	case interface{ Cause() error }:
 		if cause := err.Cause(); cause != nil {
 			e.ErrorDetails.Cause = append(e.ErrorDetails.Cause, cause)

error_go120_test.go

@@ -0,0 +1,44 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//go:build go1.20
+// +build go1.20
+
+package apm_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"go.elastic.co/apm/v2"
+	"go.elastic.co/apm/v2/apmtest"
+)
+
+func TestErrorCauseUnwrapJoined(t *testing.T) {
+	err := errors.Join(errors.New("cause1"), errors.New("cause2"))
+	_, _, errors := apmtest.WithTransaction(func(ctx context.Context) {
+		apm.CaptureError(ctx, err).Send()
+	})
+	require.Len(t, errors, 1)
+	require.Len(t, errors[0].Exception.Cause, 2)
+	assert.Equal(t, "cause1", errors[0].Exception.Cause[0].Message)
+	assert.Equal(t, "cause2", errors[0].Exception.Cause[1].Message)
+}