Add comment to netstats proto

    repeated NetworkInterfaceProto active_uid_interfaces = 2;

    // Device level network stats, which may include non-IP layer traffic.

    NetworkStatsRecorderProto dev_stats = 3;

    // IP-layer traffic stats.

    NetworkStatsRecorderProto xt_stats = 4;

    // Per-UID network stats.

    NetworkStatsRecorderProto uid_stats = 5;

    // Per-UID, per-tag network stats, excluding the default tag (i.e. tag=0).

    NetworkStatsRecorderProto uid_tag_stats = 6;

}