@Override
    public byte[] copyBuffer() {
        final byte[] copy = new byte[offset];
        System.arraycopy(buffer, 0, copy, 0, offset);
        return copy;
    }
Be careful that if internal buffer's length is as same as offset,
then just return internal buffer without copying memory for improving performance.
@return
commons-buffer/src/main/java/com/navercorp/pinpoint/common/buffer/FixedBuffer.java