add PrettyPrintf method to ObservedLogs to logger.NewTesting

ObservedLogs.PrettyPrintf is a helper function to pretty print the logs in the observedLogs

internal/pkg/agent/application/upgrade/watcher_test.go

@@ -278,14 +278,7 @@ func TestWatcher_AgentErrorQuick(t *testing.T) {
 	log, obs := logger.NewTesting("watcher")
 	defer func() {
 		if t.Failed() {
-			rawLogs := obs.All()
-			for _, rawLog := range rawLogs {
-				msg := fmt.Sprintf("[%s] %s", rawLog.Level, rawLog.Message)
-				for k, v := range rawLog.ContextMap() {
-					msg += fmt.Sprintf("%s=%v", k, v)
-				}
-				t.Log(msg)
-			}
+			obs.PrettyPrintf(t.Log)
 		}
 	}()
 	w := NewAgentWatcher(errCh, log, 100*time.Millisecond)

pkg/core/logger/testing.go

@@ -5,22 +5,42 @@
 package logger
 
 import (
+	"fmt"
+
 	"go.uber.org/zap"
 	"go.uber.org/zap/zapcore"
 	"go.uber.org/zap/zaptest/observer"
 
 	"github.com/elastic/elastic-agent-libs/logp"
 )
 
+type ObservedLogs struct {
+	*observer.ObservedLogs
+}
+
 // NewTesting creates a testing logger that buffers the logs in memory and
 // logs in debug level. Check observer.ObservedLogs for more details.
-func NewTesting(name string) (*Logger, *observer.ObservedLogs) {
+func NewTesting(name string) (*Logger, *ObservedLogs) {
 	core, obs := observer.New(zapcore.DebugLevel)
 
 	logger := logp.NewLogger(
 		name,
 		zap.WrapCore(func(in zapcore.Core) zapcore.Core {
 			return zapcore.NewTee(in, core)
 		}))
-	return logger, obs
+
+	return logger, &ObservedLogs{obs}
+}
+
+// PrettyPrintf consumes, formats and prints all captured log messages using the provided
+// print function. Therefore, after calling it, the ObservedLogs will be empty.
+func (obs *ObservedLogs) PrettyPrintf(print func(a ...any)) {
+	rawLogs := obs.TakeAll()
+	for _, rawLog := range rawLogs {
+		msg := fmt.Sprintf("[%s] %s", rawLog.Level, rawLog.Message)
+		for k, v := range rawLog.ContextMap() {
+			msg += fmt.Sprintf(" %s=%v", k, v)
+		}
+		print(msg)
+	}
 }

pkg/core/logger/testing_test.go

@@ -0,0 +1,46 @@
+package logger
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestObservedLogs_PrettyPrintf(t *testing.T) {
+	want := `
+[debug] a debug message
+[debug] a debug message with keys key2=42
+[info] an info message
+[info] an info message with keys key1=value1
+[warn] a warn message
+[warn] a warn message with keys key2=42
+[error] an error message
+[error] an error message with keys key1=value1
+`
+
+	logger, obs := NewTesting("testLogger")
+
+	logger.Debug("a debug message")
+	logger.Debugw("a debug message with keys", "key2", 42)
+	logger.Infow("an info message")
+	logger.Infow("an info message with keys", "key1", "value1")
+	logger.Warn("a warn message")
+	logger.Warnw("a warn message with keys", "key2", 42)
+	logger.Error("an error message")
+	logger.Errorw("an error message with keys", "key1", "value1")
+
+	var got strings.Builder
+	printFn := func(a ...any) {
+		_, _ = fmt.Fprintln(&got, a...)
+	}
+
+	obs.PrettyPrintf(printFn)
+	assert.Equal(t, want[1:], got.String())
+
+	// second call should be empty because PrettyPrintf should consume all logs
+	got.Reset()
+	obs.PrettyPrintf(printFn)
+	assert.Empty(t, got.String())
+}