It is meaningful for user to check the stats of IPFIX.
Using IPFIX stats, user can know how much flows the system
can support. It is also can be used for performance check
of IPFIX.

IPFIX stats is added for per IPFIX exporter. If bridge IPFIX is
enabled on the bridge, the whole bridge will have one exporter.
For flow IPFIX, the system keeps per id (column in
Flow_Sample_Collector_Set) per exporter.

1) Add 'ovs-ofctl dump-ipfix-bridge SWITCH' to export IPFIX stats of
   the bridge which enable bridge IPFIX. The output format:
   NXST_IPFIX_BRIDGE reply (xid=0x2):
     bridge ipfix: flows=0, current flows=0, sampled pkts=0, \
                   ipv4 ok=0, ipv6 ok=0, tx pkts=0
                   pkts errs=0, ipv4 errs=0, ipv6 errs=0, tx errs=0
2) Add 'ovs-ofctl dump-ipfix-flow SWITCH' to export IPFIX stats of
   the bridge which enable flow IPFIX. The output format:
   NXST_IPFIX_FLOW reply (xid=0x2): 2 ids
     id   1: flows=4, current flows=4, sampled pkts=14, ipv4 ok=13, \
             ipv6 ok=0, tx pkts=0
             pkts errs=0, ipv4 errs=0, ipv6 errs=0, tx errs=0
     id   2: flows=0, current flows=0, sampled pkts=0, ipv4 ok=0, \
             ipv6 ok=0, tx pkts=0
             pkts errs=0, ipv4 errs=0, ipv6 errs=0, tx errs=0

flows: the number of total flow records, including those exported.
current flows: the number of current flow records cached.
sampled pkts: Successfully sampled packet count.
ipv4 ok: successfully sampled IPv4 flow packet count.
ipv6 ok: Successfully sampled IPv6 flow packet count.
tx pkts: the count of IPFIX exported packets sent  to the collector(s).
pkts errs: count of packets failed when sampling, maybe not supported or other error.
ipv4 errs: Count of IPV4 flow packet in the error packets.
ipv6 errs: Count of IPV6 flow packet in the error packets.
tx errs: the count of IPFIX exported packets failed when sending to the collector(s).

Signed-off-by: Benli Ye <daniely@vmware.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>