Implement first QKD metric

[bushsf]

For #9, I think the highest priority metrics would be key-rate and key-buffer-entropy. Perhaps implementing just one or two metrics first and allowing us to test would be a good way to get started with the metrics. I don’t know the best approach for implementing it, but I’m wondering whether it should be a stand-alone tool or integrated with the proxy, e.g. “/dev/ttyS | qix-monitor | proxy.“ A qix-monitor would allow more flexibility to insert monitoring.

            qix-monitor -o output-file -f JSON|XML