Nice! I am glad my rather incohesive drivel helped.
I must point out though, my comment was about connecting to a datadog-agent over Fly’s internal network (aka 6pn), while the glimpses of your solution suggests that you’re connecting to it over the public internet (<datadog-agent-app>.fly.dev:8125 instead of top1.nearest.of.<datadog-agent-app>.internal:8125).
You might want to pass DD_API_KEY either as a build-time secret or as a runtime secret (Fly supports both).