[Discuss-gnuradio] DSP Block with internal variable output / monitor

I'd like to monitor an internal variable on a DSP block. have not made a custom block before but have read through some tutorials. 

Since the DSP inputs and outputs are vectors, and the variable is a single item whose value is dependent on the processing of the vector, the number of outputs does not match the number of inputs. The value is updated after each input vector is processed. Does this mean I should use the message port to asynchronously output the variable, or can I simply add another port and output the value as say a float or integer that is synchronously updated with the block?

For example, in a Reed-Solomon implementation, how could I add an output to monitor the number of errors corrected?