code: public static CounterId fromInt(int n)
      {
          long lowBits = 0xC000000000000000L | n;
          return new CounterId(ByteBuffer.allocate(16).putLong(0, 0).putLong(8, lowBits));
      }
docs: Function for test purposes, do not use otherwise.
  Pack an int in a valid CounterId so that the resulting ids respects the
  numerical ordering. Used for creating handcrafted but easy to
  understand contexts in unit tests (see CounterContextTest).