Support FWMark services in ipvs collector

[bazuchan]

error gathering metrics: 12 error(s) occurred:

• [from Gatherer Add runit exporter #2] collected metric "node_ipvs_backend_connections_active" { label:<name:"local_address" value:"" > label:<name:"local_port" value:"0" > label:<name:"proto" value:"FWM" > label:<name:"remote_address" value:"100.99.3.88" > label:<name:"remote_port" value:"443" > gauge:<value:0 > } was collected before with the same name and label values
• [from Gatherer Add runit exporter #2] collected metric "node_ipvs_backend_connections_inactive" { label:<name:"local_address" value:"" > label:<name:"local_port" value:"0" > label:<name:"proto" value:"FWM" > label:<name:"remote_address" value:"100.99.3.88" > label:<name:"remote_port" value:"443" > gauge:<value:0 > } was collected before with the same name and label values

# tail -n11 /proc/net/ip_vs
FWM  00003DA3 rr 
  -> 64630534:01BB      Tunnel  1      0          0         
  -> 64630358:01BB      Tunnel  1      0          0         
FWM  0000008B rr 
  -> 64630450:1447      Tunnel  1      0          0         
FWM  00001CFB rr 
  -> 64630358:0050      Tunnel  1      0          0         
  -> 64630534:0050      Tunnel  1      0          0         
FWM  000000DA rr 
  -> 64630534:0050      Tunnel  1      0          0         
  -> 64630358:0050      Tunnel  1      0          0

[SuperQ]

Looks like this was close to being fixed in #764, but the author abandoned the PR without finishing up the fixes.

[rexagod]

Can be closed.