Add missing Node SNAT information to Antrea flow data

[antoninbas]

Describe the problem/challenge you have
The Antrea FlowExporter includes Egress SNAT information in exported flow records, when an Egress SNAT policy is applied to the source Pod and the destination is external. The relevant Information Elements are: egressName, egressIP, egressNodeName, egressUUID, egressNodeUUID.

However for Pod-to-External flows which use the "default" Node SNAT, we do not include the SNAT IP. This can make it difficult for an external collector to correlate Antrea flow records to records collected by the underlay network or at the destination.

Describe the solution you'd like
I would like a new Information Element to be introduced, e.g, nodeSnatIP, which will be set to the SNAT IP used when masquerading Pod-to-External traffic. The element will be unset / empty for 1) non Pod-to-External flows, 2) Pod-to-External flows using Egress SNAT, 3) Pod-to-External flows which do not use SNAT (typically, that means Antrea is configured with noSNAT: true).

For reference, this is the code responsible for adding the Node SNAT iptables rule:

antrea/pkg/agent/route/route_linux.go

Lines 1248 to 1260 in 3c81b27

	if !c.noSNAT {
	rule := []string{
	"-A", antreaPostRoutingChain,
	"-m", "comment", "--comment", `"Antrea: masquerade Pod to external packets"`,
	"-s", podCIDR.String(), "-m", "set", "!", "--match-set", podIPSet, "dst",
	"!", "-o", c.nodeConfig.GatewayConfig.Name,
	"-j", iptables.MasqueradeTarget,
	}
	if c.nodeSNATRandomFully {
	rule = append(rule, "--random-fully")
	}
	writeLine(iptablesData, rule...)
	}

And this is an example rule:

-A ANTREA-POSTROUTING -m comment --comment "Antrea: masquerade Pod to external packets" -s 172.16.10.0/24 -m set ! --match-set ANTREA-POD-IP dst ! -o antrea-gw0 -j MASQUERADE

Note that Antrea itself does not know which SNAT IP will be used: it depends on routing and which egress interface is used. As a result, a comprehensive implementation of this feature is likely to look like #7483: the FlowExporter needs to get the information from the default conntrack zone.