code: public TOTPBuilder digits(int digits) {
          Preconditions.checkArgument(Range.closed(MIN_ALLOWED_DIGITS, MAX_ALLOWED_DIGITS).contains(digits));
          this.digits = digits;
          return this;
      }
docs: Returns this {@code TOTPBuilder} instance initialized with the specified
  {@code digits}.
  
  @param digits
  the number of digits the generated TOTP value should contain
  (must be between {@link #MIN_ALLOWED_DIGITS} and
  {@link #MAX_ALLOWED_DIGITS} inclusive)
  
  @return this {@code TOTPBuilder} instance initialized with the specified
  {@code digits}.
  
  @throws IllegalArgumentException
  if {@code digits} is not in [{@link #MIN_ALLOWED_DIGITS},
  {@link #MAX_ALLOWED_DIGITS}].