Reduce remote metrics chatter

[xxxserxxx]

gotop (optionally) exports sensor data as Prometheus metrics. It uses the default http format, which is K/V pairs with copious comments. An improvement would filter outgoing metrics to only gotop sensor data (this alone would halve -- or more -- the data transfered and parsed), and removing the comments would halve it further. An additional improvement would be to talk using a binary format, which would compress the data transfered even further as well as optimize data consumption.

[xxxserxxx]

VictoriaMetrics solves most of these issues, and HTTP/2 may solve the rest.

• HTTP/2 compression may reduce the benefit to using a binary format to the point of being not worth the effort
• HTTP/2 streams can further reduce the HTTP call overhead
• VictoriaMetrics Sets can provide the desire to limit output to only gotop metrics

I'm changing this ticket to mean: implement VictoriaMetrics Sets; the HTTP/2 work will have its own ticket.