Kristjan/container

[kristjanvalur]

Description

Add support for the DD_EXTERNAL_ENV, DD_ENTITY_ID and DD_CONTAINER_ID configurables

Motivation

Other implementations, such as dd-trace-py, already support these, which help with running tracing from containerized
environments. The Datadog Admission Controller sets DD_EXTERNAL_ENV and DD_ENTITY_ID, and the DD_CONTAINER_ID can be provided by the user if he has previously discovered the ID using an external script.

Additional Notes

• See issue [FEATURE]: Support origin discovery via entity-id or container-id #193
• No attempt is made at automatic discovery of container id, such as is done in the python version, due to the relative complexity of performing grep and such analysis from C, when simpler solutions exists in the form of Bash.
• An example container-id.sh is added to examples/container showing how to populate DD_CONTAINER_ID, based on existing code from other Datadog libraries in Python.
• This is based on analyzing dd-trace-py and some cursory glancing at datadog-agent. I may have got things wrong. For instance, dd-trace-py sets a Datadog-Entity-ID header (in addtion to the tag) while the agent does not parse any such header, so we skip setting that header in this implementation.
• The capitalization of the header fields follows what was found in dd-trace-py, not any official internal specifications
• I have not been able to fully run the test suite, so the Unit tests provided may or may not work :)

Jira ticket: [PROJ-IDENT]

[dmehala]

Closing because it's been implemented in #214.
Thanks a lot @kristjanvalur for your contribution and your patience.