Add aggregated NIC stats

This change also includes miscellaneous improvements:
* UnknownProtocolRcvdPackets has been separated into two stats, to
specify at which layer the unknown protocol was found (L3 or L4)
* MalformedRcvdPacket is not aggregated across every endpoint anymore.
Doing it this way did not add useful information, and it was also error-prone
(example: ipv6 forgot to increment this aggregated stat, it only
incremented its own ipv6.MalformedPacketsReceived). It is now only incremented
 the NIC.
* Removed TestStatsString test which was outdated and had no real
utility.

PiperOrigin-RevId: 375057472

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