Add Datadog to devstack, available by default and inactive by default

[jristau1984]

Follow up from 1/24 retro action item: Devstack should have Datadog pre-configured by default, but not active by default. Activating DataDog should be as easy as flipping a switch.

[robrap]

I moved this to Datadog Improvements epic. It could live in either, but it really enhances the dev experience specifically for someone working on Datadog Improvements.

[robrap]

Taegyun from DD provided the following, taken from this past github comment:

https://github.com/taegyunkim/devstack/tree/taegyunkim/datadog-local-testing

I have added datadog agent as https://github.com/taegyunkim/devstack/blob/a5a54f6de732fc504259d3f388803391234fc639/docker-compose.yml#L455

Additional notes:

• This is the direction we want to go.
• If there is an initial iteration where we could quickly commit this with a doc on how to update the DD environment variables and a big warning not to commit your API keys, we should do that.
• @timmc-edx also worked on a branch for a more permanent solution to setting up the DD environment variables (which he can share), but this would take some additional work to get right and may be an additional iteration on the solution.

[timmc-edx]

Here's my partial branch for integrating a datadog-agent container: edx/devstack@timmc/datadog-local-testing...edx:devstack:timmc/dd-agent-experiment

The important part is that it would use env_file to allow keeping the API key (and hostname) environment variables in a private, unversioned file so that there's no risk of pushing them to GitHub. I didn't get as far as making this all work, but you can see where I was going.

[robrap]

@papphelix: This ticket is mainly useful for enhancing our supporting tools for Datadog, which is mainly arch-bom related work, and why this is on our backlog. I'm going to move this back to the Backlog and remove you as an assignee, and we can discuss further in Slack with @vgulati-apphelix. Thank you.