static String toLowerHex(long v) {
    char[] data = RecyclableBuffers.shortStringBuffer();
    writeHexLong(data, 0, v);
    return new String(data, 0, 16);
  }
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