Fixup metrics for socket_refused_connections

[hamiltont]

systemd_socket_refused_connections_total is named as though it's a counter. However, it's not a counter, it's a gauge. Either rename or internally make it a counter

[povilasv]

Looks like this is also an issue in Node exporter

https://github.com/prometheus/node_exporter/blob/2477c5c67dff7e7655a9d466450235e9c9eac193/collector/systemd_linux.go#L306-L308

[povilasv]

I think it needs to be a counter not an gauge

as looking at the code, stuff gets incremented:

https://github.com/systemd/systemd/blob/de5ae832f26cc9b049ee28b61df816669a69e306/src/core/socket.h#L70
https://github.com/systemd/systemd/blob/7c286cd6a615fa9bce8a2830133bcf89becfbf9f/src/core/socket.c#L2659

[povilasv]

Currently PR in progress #10