Add L4 protocol label to advanced metrics

[andreev-io]

Is your feature request related to a problem? Please describe.
Advanced metrics include packet information such as IP and port. A typical packet 5-tuple is (protocol, src IP, src port, dst IP, dst port) (this would be a 3-tuple in the Local Context mode). This issue is to add the protocol label to such plugins as dropreason and packetparser.

Describe the solution you'd like
The protocol label should be optionally included in such metrics as drop_count, drop_bytes, adv_forward_bytes, adv_forward_count.

Describe alternatives you've considered
N/A

Additional context
N/A

[andreev-io]

I opened #243 as a placeholder. While working on it, I originally wanted to add protocol configuration to pkg/module/metrics/types.go because I thought I would code protocol up as an additionalLabel metric. However, I saw that the additionalLabel configuration is actually never utilised (correct me if I'm wrong), and labels such as direction (which are said to be "additional" in the documentation) are actually set directly through the module code, e.g. in pkg/module/metrics/forward.go. Following this model, I added my code to forward.go.

Should there be a separate issue for making additionalLabel configuration work?

As for this change, am I approaching it correctly?

[vakalapa]

Yeah the idea was to use additional labels field for optional labels we might need later on. But for this protocol field, current approach makes perfect sense.