Fields of type gadget_l3endpoint_t not displayed correctly in both columns and json outputs

[alban]

Description

I am writing a gadget using struct gadget_l3endpoint_t (see #2795 and I have some problem where the endpoints columns are not displayed correctly:

1. The column ENDPOINT.STRING looks good but the column ENDPOINT.ADDR.V6 looks mangled.

$ sudo IG_EXPERIMENTAL=true ig run albantest.azurecr.io/trace_udns:t1 --verify-image=false --host
INFO[0000] Experimental features enabled                
WARN[0000] you set --verify-image=false, image will not be verified 
WARN[0002] you set --verify-image=false, image will not be verified 
RUNTIME.CONTAINERNAME                    MNTNS_ID         PID                   TID            COMM             NAME                  SERVICE               ENDPOINT.ADDR.V6      ENDPOINT.ADDR… EN… OPERATION_STR        ENDPOINT.STRING     
                                         4026531841       3326691               3326692        curl             www.wikipedia.org     80                    �;�                  3761967033     4   getaddrinfo          185.15.59.224

2. The json output does not show the endpoint at all:

$ sudo IG_EXPERIMENTAL=true ig run albantest.azurecr.io/trace_udns:t1 --verify-image=false --host -o json
INFO[0000] Experimental features enabled                
WARN[0000] you set --verify-image=false, image will not be verified 
WARN[0002] you set --verify-image=false, image will not be verified 
{"comm":"curl","k8s":{"container":"","hostnetwork":false,"namespace":"","node":"","pod":""},"mntns_id":4026531841,"name":"www.wikipedia.org","operation":0,"operation_str":"getaddrinfo","pid":3337126,"runtime":{"containerId":"","containerImageDigest":"","containerImageName":"","containerName":"","runtimeName":""},"service":"80","tid":3337127}

Impact

Gadget authors cannot use gadget_l3endpoint_t in their gadgets.

Environment and steps to reproduce

See above.

Expected behavior

• When an IP is IPv4, the column ENDPOINT.ADDR.V6 should not display anything.
• The json output should include the endpoints.

Additional information

cc @flyth