Add support of ExtendedMPLS for sflow

[akarneliuk]

Is your feature request related to a problem? Please describe.
the sflow v5 has implemented extended mpls struct, which contains mpls data. This is not implemented in goflow2 sflow. As such, i cannot properly decode mpls labels

Describe the solution you'd like
To add support of mpls extended mpls to sflow part of goflow2 to make this information available upon parsing.

Describe alternatives you've considered
There is not much alternative here. I am. Using telegraf, which relies in this library for sflow parsing.

Additional context
https://sflow.org/SFLOW-STRUCTS5.txt

[lspgn]

Thank you @akarneliuk for the suggestion. This should be possible. GoFlow2 already decodes MPLS so all the fields already exist in the protobuf.

Although: what is the model of your device producing sFlow? Have you tried with regular Ethernet samples?
Do you have a packet capture?