structured logging: indent multi-line string with tab

This makes it possible to distinguish between value (indended with tab) and the
final " <<<" delimiter (indended with space).

klog.go

@@ -878,7 +878,7 @@ func writeStringValue(b *bytes.Buffer, quote bool, v string) {
 	// Complex multi-line string, show as-is with indention.
 	b.WriteString(">>>\n")
 	for index != -1 {
-		b.WriteByte(' ')
+		b.WriteByte('\t')
 		b.Write(data[0 : index+1])
 		data = data[index+1:]
 		index = bytes.IndexByte(data, '\n')
@@ -889,7 +889,7 @@ func writeStringValue(b *bytes.Buffer, quote bool, v string) {
 	} else {
 		// No line break at end of last line, write rest of string and
 		// add one.
-		b.WriteByte(' ')
+		b.WriteByte('\t')
 		b.Write(data)
 		b.WriteString("\n <<<")
 	}

klog_test.go

@@ -973,8 +973,8 @@ with a line break.`,
 			msg: `first message line
 second message line`,
 			format: `I0102 15:04:05.067890    1234 klog_test.go:%d] "first message line\nsecond message line" multiLine>>>
- first value line
- second value line
+	first value line
+	second value line
  <<<
 `,
 			keysValues: []interface{}{"multiLine", `first value line
@@ -983,8 +983,8 @@ second value line`},
 		{
 			msg: `message`,
 			format: `I0102 15:04:05.067890    1234 klog_test.go:%d] "message" myData>>>
- {Data:This is some long text
- with a line break.}
+	{Data:This is some long text
+	with a line break.}
  <<<
 `,
 			keysValues: []interface{}{"myData", myData},
@@ -1109,10 +1109,10 @@ No whitespace.`,
 				"pod", "kubedns",
 			},
 			want: ` multiLineString>>>
- Hello world!
- 	Starts with tab.
-   Starts with spaces.
- No whitespace.
+	Hello world!
+		Starts with tab.
+	  Starts with spaces.
+	No whitespace.
  <<< pod="kubedns"`,
 		},
 		{