Add container labels field from ECS

[ChrsMark]

This PR is part of #72 and adds the container.lables.<key> similarly to http-request-and-response-headers.

Note that these labels refer to the container labels themselves and not the container's image labels. For image labels we would need to attach them under container.image.labels.* instead but for now there is no specific need for populating these too.

Note also that I had to manually update the markdown table cause the make table-generation command was failing with the following:

Invalid id labels.<key>. Semantic Convention ids MUST match ([a-z](\.?[a-z0-9_-]+)+) - @35:9 make: *** [Makefile:77: table-generation] Error 1

It seems that the http-headers table was created manually too.

cc: @AlexanderWert @kaiyan-sheng @mlunadia

[AlexanderWert]

LGTM

[ChrsMark]

@open-telemetry/specs-semconv-approvers @open-telemetry/specs-semconv-maintainers could we have some more reviews on this? Otherwise we could merge if no objection?

[ChrsMark]

Thanks for the reviews folks! Anything else missing here?

[joaopgrassi]

@ChrsMark I just looked into https://www.elastic.co/guide/en/ecs/current/ecs-container.html#field-container-labels and there it says: "Image labels.". But in the PR here you say these are container labels, and not image. Is this intended? Wouldn't that contradict ECS? 🤔

[kaiyan-sheng]

@ChrsMark I just looked into https://www.elastic.co/guide/en/ecs/current/ecs-container.html#field-container-labels and there it says: "Image labels.". But in the PR here you say these are container labels, and not image. Is this intended? Wouldn't that contradict ECS? 🤔

@joaopgrassi Sorry Christos is out on vacation this week. I think in ECS Image labels is specifically talking about docker container labels. These labels are added to the image through dockerfile and then to the container. So here for the container labels, it doesn't contradict ECS.