Add ARP collector for Linux (#540)

* Implement commonalities and linux support for ARP collection

* Add ARP collector to fixtures and run as part of e2e tests

* Bubble up scanner errors

* Use single return values where it makes sense

* Add missing annotation

* Move arp_common into arp_linux

* Add license header to arp_linux.go

* Address initial feedback

* Use strings.Fields instead of strings.Split

* Deal with scanner.Err() rather than throwing away errors

* Check for scan errors in-line before interacting with the entries map

* Don't interact with potentially empty text from scan

* Check for scan errors outside the scan loop

* Add comment about moving procfs parsing

* Add more direct comment

* Update initialism style to match go style guide

* Put function args on the same line

* Add TODO in front of comment about procfs extraction

* Guard against strings.Fields returning an empty slice

* Be more defensive about ARP table format and use upcase more broadly

* Enable the ARP collector by default

* Add ARP collector to the README

* Remove 'entry'

end-to-end-test.sh

@@ -3,6 +3,7 @@
 set -euf -o pipefail
 
 collectors=$(cat << COLLECTORS
+  arp
   buddyinfo
   conntrack
   diskstats