feat: Support container ID field (dogstatsd 1.2)

[ahmed-mez]

The PR does the following

• Adds c:<id> field holding the container ID so the agent can attach container tags.

Brief explanation of how the new container field is handled

• If DD_ENTITY_ID is set, we ignore the container field
• If DD_ENTITY_ID is not set and DD_ORIGIN_DETECTION_ENABLED is not explicitly set to false or the WithoutOriginDetection() is not invoked, we try to get the app container ID by parsing /proc/self/cgroup. In case of success, the container field in will be set to .

Motivation

• More granular tags (container tags instead of pod tags)
• Works for containers outside of Kubernetes (e.g ECS, Fargate)
• The goal of not reusing dd.internal.entity_id is to avoid parsing the value and guess the prefix on the agent side (avoid costly operations in the hot path)

Notes

• Users should explicitly enable container tagging in the agent otherwise the agent ignores the container field. So even if the client sends the field automatically, the tagging won't be done without explicitly enabling it in the agent.
• Cgroup parsing logic is essentially the same as in dd-trace-go
• This feature requires an agent change [dsd] Support container ID field from dogstatsd 1.2 datadog-agent#10659
• Replaces [dogstatsd] Add support for including container id #172

[hush-hush]

I added a few comments. We should wait on the final decision between tags and new field in dogstatsd protocol before going further.

[hush-hush]

I think it's almost good to go.

[hush-hush]

Thanks for the PR !

[ahmed-mez]

Thank you for the review @hush-hush ! I'd like to get 👀 on the Agent PR before merging this one.

PTAL when you get a chance! DataDog/datadog-agent#10659