arp: optimize netlink interface name resolution

github.com/jsimonetti/rtnetlink provides a high level rtnl wrapper
around the lower level rtnetlink functions, which essentially does all
that we need. The rtnl.Conn.Neighbors uses an internal cache for
resolving interface indexes to names, so it makes at most one rtnetlink
call per interface to resolve the name.

Using this high level wrapper hugely simplifies our code and makes it
easier to understand and maintain.

Fixes: #3075

Signed-off-by: Daniel Swarbrick <daniel.swarbrick@gmail.com>

collector/arp_linux.go

@@ -17,13 +17,11 @@
 package collector
 
 import (
-	"errors"
 	"fmt"
 	"log/slog"
-	"net"
 
 	"github.com/alecthomas/kingpin/v2"
-	"github.com/jsimonetti/rtnetlink/v2"
+	"github.com/jsimonetti/rtnetlink/v2/rtnl"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/procfs"
 	"golang.org/x/sys/unix"
@@ -76,44 +74,30 @@ func getTotalArpEntries(deviceEntries []procfs.ARPEntry) map[string]uint32 {
 }
 
 func getTotalArpEntriesRTNL() (map[string]uint32, error) {
-	conn, err := rtnetlink.Dial(nil)
+	conn, err := rtnl.Dial(nil)
 	if err != nil {
 		return nil, err
 	}
 	defer conn.Close()
 
-	neighbors, err := conn.Neigh.List()
+	// Neighbors will also contain IPv6 neighbors, but since this is purely an ARP collector,
+	// restrict to AF_INET.
+	neighbors, err := conn.Neighbours(nil, unix.AF_INET)
 	if err != nil {
 		return nil, err
 	}
 
-	ifIndexEntries := make(map[uint32]uint32)
+	// Map of interface name to ARP neighbor count.
+	entries := make(map[string]uint32)
 
 	for _, n := range neighbors {
-		// Neighbors will also contain IPv6 neighbors, but since this is purely an ARP collector,
-		// restrict to AF_INET. Also skip entries which have state NUD_NOARP to conform to output
-		// of /proc/net/arp.
-		if n.Family == unix.AF_INET && n.State&unix.NUD_NOARP == 0 {
-			ifIndexEntries[n.Index]++
-		}
-	}
-
-	enumEntries := make(map[string]uint32)
-
-	// Convert interface indexes to names.
-	for ifIndex, entryCount := range ifIndexEntries {
-		iface, err := net.InterfaceByIndex(int(ifIndex))
-		if err != nil {
-			if errors.Unwrap(err).Error() == "no such network interface" {
-				continue
+		// Skip entries which have state NUD_NOARP to conform to output of /proc/net/arp.
+		if n.State&unix.NUD_NOARP == 0 {
+			entries[n.Interface.Name]++
 		}
-			return nil, err
-		}
-
-		enumEntries[iface.Name] = entryCount
 	}
 
-	return enumEntries, nil
+	return entries, nil
 }
 
 func (c *arpCollector) Update(ch chan<- prometheus.Metric) error {