wgengine: add exit destination logging enable for wgengine logger (#11952)

Updates tailscale/corp#18625
Co-authored-by: Kevin Liang <kevinliang@tailscale.com>
Signed-off-by: Claire Wang <claire@tailscale.com>

wgengine/wgcfg/config.go

@@ -27,9 +27,11 @@ type Config struct {
 
 	// NetworkLogging enables network logging.
 	// It is disabled if either ID is the zero value.
+	// LogExitFlowEnabled indicates whether or not exit flows should be logged.
 	NetworkLogging struct {
-		NodeID   logid.PrivateID
-		DomainID logid.PrivateID
+		NodeID             logid.PrivateID
+		DomainID           logid.PrivateID
+		LogExitFlowEnabled bool
 	}
 }