structured logging: avoid quoting of strings with line breaks

It makes the output harder to read, in particular when the string is long (see
kubernetes/kubernetes#104868).

The revised handling of key/value pairs will produce the following output for
two key/value pairs where the first one has a multi-line string as value:

 multiLineString=vvvvv string vvvvv
 Hello world!
 	Starts with tab.
   Starts with spaces.
 No whitespace.
 ^^^^^ string ^^^^^
 pod="kubedns"

klog.go

@@ -824,21 +824,37 @@ func kvListFormat(b *bytes.Buffer, keysAndValues ...interface{}) {
 		}
 		b.WriteByte(' ')
 
-		switch v.(type) {
-		case string, error:
+		switch v := v.(type) {
+		case string:
+			writeStringValue(b, k, v)
+		case error:
 			b.WriteString(fmt.Sprintf("%s=%q", k, v))
 		case []byte:
 			b.WriteString(fmt.Sprintf("%s=%+q", k, v))
 		default:
-			if _, ok := v.(fmt.Stringer); ok {
-				b.WriteString(fmt.Sprintf("%s=%q", k, v))
+			if s, ok := v.(fmt.Stringer); ok {
+				writeStringValue(b, k, s.String())
 			} else {
 				b.WriteString(fmt.Sprintf("%s=%+v", k, v))
 			}
 		}
 	}
 }
 
+func writeStringValue(b *bytes.Buffer, k interface{}, v string) {
+	if !strings.Contains(v, "\n") {
+		// Simple string, quote quotation marks and non-printable characters.
+		b.WriteString(fmt.Sprintf("%s=%q", k, v))
+		return
+	}
+	// Complex multi-line string, show as-is with indention.
+	b.WriteString(fmt.Sprintf("%s=vvvvv string vvvvv\n", k))
+	for _, line := range strings.Split(v, "\n") {
+		b.WriteString("  " + line + "\n")
+	}
+	b.WriteString("  ^^^^^ string ^^^^^")
+}
+
 // redirectBuffer is used to set an alternate destination for the logs
 type redirectBuffer struct {
 	w io.Writer

klog_test.go

@@ -1075,6 +1075,20 @@ func TestKvListFormat(t *testing.T) {
 			keysValues: []interface{}{"pod", "kubedns", "bytes", []byte("��=� ⌘")},
 			want:       " pod=\"kubedns\" bytes=\"\\ufffd\\ufffd=\\ufffd \\u2318\"",
 		},
+		{
+			keysValues: []interface{}{"multiLineString", `Hello world!
+	Starts with tab.
+  Starts with spaces.
+No whitespace.`,
+				"pod", "kubedns",
+			},
+			want: ` multiLineString=vvvvv string vvvvv
+  Hello world!
+  	Starts with tab.
+    Starts with spaces.
+  No whitespace.
+  ^^^^^ string ^^^^^ pod="kubedns"`,
+		},
 		{
 			keysValues: []interface{}{"pod", "kubedns", "maps", map[string]int{"three": 4}},
 			want:       " pod=\"kubedns\" maps=map[three:4]",