logtest: add Transform (#6794)

Fixes https://github.com/open-telemetry/opentelemetry-go/issues/6491
Fixes https://github.com/open-telemetry/opentelemetry-go/issues/6489

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

Per
https://github.com/open-telemetry/opentelemetry-go/pull/6774#discussion_r2091424563

CHANGELOG.md

@@ -22,6 +22,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Add `Values` method to `HeaderCarrier` to implement the new `ValuesGetter` interface in `go.opentelemetry.io/otel/propagation`. (#5973)
 - Update `Baggage` in `go.opentelemetry.io/otel/propagation` to retrieve multiple values for a key when the carrier implements `ValuesGetter`. (#5973)
 - Add `AssertEqual` function in `go.opentelemetry.io/otel/log/logtest`. (#6662)
+- Add `Transform` option in `go.opentelemetry.io/otel/log/logtest`. (#6794)
 
 ### Removed
 

log/logtest/assert.go

@@ -25,11 +25,16 @@ type testingT interface {
 	Errorf(format string, args ...any)
 }
 
-func assertEqual[T Recording | Record](t testingT, want, got T, _ ...AssertOption) bool {
+func assertEqual[T Recording | Record](t testingT, want, got T, opts ...AssertOption) bool {
 	if h, ok := t.(interface{ Helper() }); ok {
 		h.Helper()
 	}
 
+	var cfg assertConfig
+	for _, opt := range opts {
+		cfg = opt.apply(cfg)
+	}
+
 	cmpOpts := []cmp.Option{
 		cmp.Comparer(func(x, y context.Context) bool { return x == y }), // Compare context.
 		cmpopts.SortSlices(
@@ -37,6 +42,7 @@ func assertEqual[T Recording | Record](t testingT, want, got T, _ ...AssertOptio
 		), // Unordered compare of the key values.
 		cmpopts.EquateEmpty(), // Empty and nil collections are equal.
 	}
+	cmpOpts = append(cmpOpts, cfg.cmpOpts...)
 
 	if diff := cmp.Diff(want, got, cmpOpts...); diff != "" {
 		t.Errorf("mismatch (-want +got):\n%s", diff)
@@ -45,9 +51,27 @@ func assertEqual[T Recording | Record](t testingT, want, got T, _ ...AssertOptio
 	return true
 }
 
-type assertConfig struct{}
+type assertConfig struct {
+	cmpOpts []cmp.Option
+}
 
 // AssertOption allows for fine grain control over how AssertEqual operates.
 type AssertOption interface {
 	apply(cfg assertConfig) assertConfig
 }
+
+type fnOption func(cfg assertConfig) assertConfig
+
+func (fn fnOption) apply(cfg assertConfig) assertConfig {
+	return fn(cfg)
+}
+
+// Transform applies a transformation f function that
+// converts values of a certain type into that of another.
+// f must not mutate A in any way.
+func Transform[A, B any](f func(A) B) AssertOption {
+	return fnOption(func(cfg assertConfig) assertConfig {
+		cfg.cmpOpts = append(cfg.cmpOpts, cmp.Transformer("", f))
+		return cfg
+	})
+}

log/logtest/assert_test.go

@@ -155,6 +155,22 @@ func TestAssertEqualRecord(t *testing.T) {
 			},
 			want: false,
 		},
+		{
+			name: "Transform to ignore timestamps",
+			a: Record{
+				Attributes: []log.KeyValue{log.Int("n", 1), log.String("foo", "bar")},
+			},
+			b: Record{
+				Timestamp:  y2k,
+				Attributes: []log.KeyValue{log.String("foo", "bar"), log.Int("n", 1)},
+			},
+			opts: []AssertOption{
+				Transform(func(time.Time) time.Time {
+					return time.Time{}
+				}),
+			},
+			want: true,
+		},
 	}
 
 	for _, tc := range tests {

log/logtest/example_test.go

@@ -0,0 +1,55 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package logtest_test
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"go.opentelemetry.io/otel/log"
+	"go.opentelemetry.io/otel/log/logtest"
+)
+
+func Example() {
+	t := &testing.T{} // Provided by testing framework.
+
+	// Create a recorder.
+	rec := logtest.NewRecorder()
+
+	// Emit a log record (code under test).
+	l := rec.Logger("Example")
+	ctx := context.Background()
+	r := log.Record{}
+	r.SetTimestamp(time.Now())
+	r.SetSeverity(log.SeverityInfo)
+	r.SetBody(log.StringValue("Hello there"))
+	r.AddAttributes(log.String("foo", "bar"))
+	r.AddAttributes(log.Int("n", 1))
+	l.Emit(ctx, r)
+
+	// Verify that the expected and actual log records match.
+	want := logtest.Recording{
+		logtest.Scope{Name: "Example"}: []logtest.Record{
+			{
+				Context:  context.Background(),
+				Severity: log.SeverityInfo,
+				Body:     log.StringValue("Hello there"),
+				Attributes: []log.KeyValue{
+					log.Int("n", 1),
+					log.String("foo", "bar"),
+				},
+			},
+		},
+	}
+	got := rec.Result()
+	logtest.AssertEqual(t, want, got,
+		// Ignore Timestamps.
+		logtest.Transform(func(time.Time) time.Time {
+			return time.Time{}
+		}),
+	)
+	// Output:
+	//
+}