StringBuilder buf = new StringBuilder();
        Supplier<Object> supplier = () -> "テスト 测试 테스트 🎉";

        monitorTarget.append(buf, "unicodeKey", supplier);
        // Unicode characters are escaped in JSON format
        assertEquals("\"unicodeKey\":\"\\u30C6\\u30B9\\u30C8 \\u6D4B\\u8BD5 \\uD14C\\uC2A4\\uD2B8 \\uD83C\\uDF89\"", buf.toString());
    }

    // Test append returns the same buffer instance

 *
 * <p>For up-to-date Unicode character properties (digit, letter, etc.) and support for
 * supplementary code points, use ICU4J UCharacter and UnicodeSet (freeze() after building). For
 * basic text processing based on UnicodeSet use the ICU4J UnicodeSetSpanner.
 *
 * <p>Example usages:
 *
 * <pre>
 *   String trimmed = {@link #whitespace() whitespace()}.{@link #trimFrom trimFrom}(userInput);

 *
 * <p>For up-to-date Unicode character properties (digit, letter, etc.) and support for
 * supplementary code points, use ICU4J UCharacter and UnicodeSet (freeze() after building). For
 * basic text processing based on UnicodeSet use the ICU4J UnicodeSetSpanner.
 *
 * <p>Example usages:
 *
 * <pre>
 *   String trimmed = {@link #whitespace() whitespace()}.{@link #trimFrom trimFrom}(userInput);