logtest: add Desc (#6796)

Fixes https://github.com/open-telemetry/opentelemetry-go/issues/6490

Fixes https://github.com/open-telemetry/opentelemetry-go/issues/6341

CHANGELOG.md

@@ -26,6 +26,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
   The package contains semantic conventions from the `v1.32.0` version of the OpenTelemetry Semantic Conventions.
   See the [migration documentation](./semconv/v1.32.0/MIGRATION.md) for information on how to upgrade from `go.opentelemetry.io/otel/semconv/v1.31.0`(#6782)
 - Add `Transform` option in `go.opentelemetry.io/otel/log/logtest`. (#6794)
+- Add `Desc` option in `go.opentelemetry.io/otel/log/logtest`. (#6796)
 
 ### Removed
 

log/logtest/assert.go

@@ -45,7 +45,16 @@ func assertEqual[T Recording | Record](t testingT, want, got T, opts ...AssertOp
 	cmpOpts = append(cmpOpts, cfg.cmpOpts...)
 
 	if diff := cmp.Diff(want, got, cmpOpts...); diff != "" {
-		t.Errorf("mismatch (-want +got):\n%s", diff)
+		msg := "mismatch (-want +got):\n%s"
+		if cfg.msg != "" {
+			msg = cfg.msg + "\n" + msg
+		}
+
+		args := make([]any, 0, len(cfg.args)+1)
+		args = append(args, cfg.args...)
+		args = append(args, diff)
+
+		t.Errorf(msg, args...)
 		return false
 	}
 	return true
@@ -53,6 +62,8 @@ func assertEqual[T Recording | Record](t testingT, want, got T, opts ...AssertOp
 
 type assertConfig struct {
 	cmpOpts []cmp.Option
+	msg     string
+	args    []any
 }
 
 // AssertOption allows for fine grain control over how AssertEqual operates.
@@ -75,3 +86,13 @@ func Transform[A, B any](f func(A) B) AssertOption {
 		return cfg
 	})
 }
+
+// Desc prepends the given text to an assertion failure message.
+// The text is formatted with the args using fmt.Sprintf.
+func Desc(text string, args ...any) AssertOption {
+	return fnOption(func(cfg assertConfig) assertConfig {
+		cfg.msg = text
+		cfg.args = args
+		return cfg
+	})
+}

log/logtest/assert_test.go

@@ -5,10 +5,12 @@ package logtest
 
 import (
 	"context"
+	"fmt"
 	"testing"
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 
 	"go.opentelemetry.io/otel/log"
 )
@@ -20,7 +22,7 @@ type mockTestingT struct {
 }
 
 func (m *mockTestingT) Errorf(format string, args ...any) {
-	m.errors = append(m.errors, format)
+	m.errors = append(m.errors, fmt.Sprintf(format, args...))
 }
 
 func TestAssertEqual(t *testing.T) {
@@ -186,3 +188,18 @@ func TestAssertEqualRecord(t *testing.T) {
 		})
 	}
 }
+
+func TestDesc(t *testing.T) {
+	mockT := &mockTestingT{}
+	a := Record{
+		Attributes: []log.KeyValue{log.String("foo", "bar")},
+	}
+	b := Record{
+		Attributes: []log.KeyValue{log.Int("n", 1)},
+	}
+
+	assertEqual(mockT, a, b, Desc("custom message, %s", "test"))
+
+	require.Len(t, mockT.errors, 1, "expected one error")
+	assert.Contains(t, mockT.errors[0], "custom message, test\n", "expected custom message")
+}