code: public int computeMessageSize(int fieldNumber, K key, V value) {
          return CodedOutputStream.computeTagSize(fieldNumber)
                  + CodedConstant.computeLengthDelimitedFieldSize(computeSerializedSize(metadata, key, value));
      }
docs: Computes the message size for the provided key and value as though they were wrapped by a {@link MapEntryLite}.
  This helper method avoids allocation of a {@link MapEntryLite} built with a key and value and is called from
  generated code directly.
  
  @param fieldNumber the field number
  @param key the key
  @param value the value
  @return the int