feat(profiler): support configurable debug logging destination (#8104)

* feat(profiler): support configurable debug logging destination

* address feedback

profiler/profiler.go

@@ -39,6 +39,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io"
 	"log"
 	"os"
 	"regexp"
@@ -130,6 +131,10 @@ type Config struct {
 	// defaults to false.
 	DebugLogging bool
 
+	// DebugLoggingOutput is where the logger will write debug logs to, if enabled.
+	// It defaults to os.Stderr.
+	DebugLoggingOutput io.Writer
+
 	// MutexProfiling enables mutex profiling. It defaults to false.
 	// Note that mutex profiling is not supported by Go versions older
 	// than Go 1.8.
@@ -225,7 +230,10 @@ func Start(cfg Config, options ...option.ClientOption) error {
 }
 
 func start(cfg Config, options ...option.ClientOption) error {
-	logger = log.New(os.Stderr, "Cloud Profiler: ", log.LstdFlags)
+	if cfg.DebugLoggingOutput == nil {
+		cfg.DebugLoggingOutput = os.Stderr
+	}
+	logger = log.New(cfg.DebugLoggingOutput, "Cloud Profiler: ", log.LstdFlags)
 	if err := initializeConfig(cfg); err != nil {
 		debugLog("failed to initialize config: %v", err)
 		return err

profiler/profiler_test.go

@@ -972,13 +972,16 @@ func TestAgentWithServer(t *testing.T) {
 	quitProfilee := make(chan bool)
 	go profileeLoop(quitProfilee)
 
+	var logs bytes.Buffer
 	if err := Start(Config{
-		Service:     testService,
-		ProjectID:   testProjectID,
-		APIAddr:     srv.Addr,
-		Instance:    testInstance,
-		Zone:        testZone,
-		numProfiles: 2,
+		Service:            testService,
+		ProjectID:          testProjectID,
+		APIAddr:            srv.Addr,
+		Instance:           testInstance,
+		Zone:               testZone,
+		numProfiles:        2,
+		DebugLogging:       true,
+		DebugLoggingOutput: &logs,
 	}); err != nil {
 		t.Fatalf("Start(): %v", err)
 	}
@@ -997,6 +1000,10 @@ func TestAgentWithServer(t *testing.T) {
 			t.Errorf("validateProfile(%s) got error: %v", pType, err)
 		}
 	}
+
+	if logs.Len() == 0 {
+		t.Error("expected some debug logging output, but got none")
+	}
 }
 
 // testConnPool is a gtransport.ConnPool used for testing.