FIX: couble count of flow_process_nf_total

[bpereto]

I was confused about the latest total processed flow counts - if I interpreting it right, the metric of flow_process_nf_total is counted double for v9 and v10:

https://github.com/netsampler/goflow2/blob/ee095a980f28a9cd9b478e12b9ff7d41ba533762/metrics/producer.go#L106

and in the recordCommonNetFlowMetrics function: https://github.com/netsampler/goflow2/blob/ee095a980f28a9cd9b478e12b9ff7d41ba533762/metrics/decoder.go#L89