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arp: optimize interface name resolution #3133

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Go net package's net.InterfaceByIndex fetches the entire interface table behind the scenes, despite us specifying the interface index that we are interested in. This results in an exponential amount of rtnetlink traffic (and thus recvfrom syscalls) as the number of interfaces in a host increases.

Fixes: #3075

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Great find! Thanks!

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: prometheus#3075

Signed-off-by: Daniel Swarbrick <daniel.swarbrick@gmail.com>
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Bug: collector.arp.netlink is super slow to retrieve metrics
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