[statsd] Add origin detection with container ID field

[ahmed-mez]

What does this PR do?

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

Description of the Change

• 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 and the origin_detection_enabled param is not explicitly disabled, we try to get the app container ID by parsing /proc/self/cgroup. In case of success, the container field in will be set.

Alternate Designs

Possible Drawbacks

Verification Process

Wrote a small app importing the lib and sending metrics to agent 7.35 with origin detection client enabled - the metrics were tagged with container tags.

Additional 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-py
• This feature requires an agent change https://github.com/DataDog/datadog-agent/pull/10659
• Reference implementation is in Go https://github.com/DataDog/datadog-go/pull/250
• Public docs https://github.com/DataDog/documentation/pull/13241

Release Notes

Support adding the container ID field for origin detection (dogstatsd 1.2)

Review checklist (to be filled by reviewers)

• [ ] Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• [ ] PR should not have do-not-merge/ label attached.
• [ ] If Applicable, issue must have kind/ and severity/ labels attached at least.

[sgnn7]

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