static String padLeft(String id, int desiredLength) {
    int length = id.length();
    int numZeros = desiredLength - length;

    char[] data = RecyclableBuffers.shortStringBuffer();
    THIRTY_TWO_ZEROS.getChars(0, numZeros, data, 0);
    id.getChars(0, length, data, numZeros);

    return new String(data, 0, desiredLength);
  }
Returns a valid lower-hex trace ID, padded left as needed to 16 or 32 characters.

@throws IllegalArgumentException if oversized or not lower-hex
zipkin/src/main/java/zipkin2/Span.java