What are the possible reasons that tracing headers are not added to request?

[gajus]

I am seeing sentry-trace and baggage headers added in development, but not in production.

I've confirmed that tracePropagationTargets matches the pattern of the request.

[gajus]

I am posting this to save others time, but TLDR is that only possibility is that you are tree-shaking the Sentry bundle.

We had __SENTRY_TRACING__ constant set in production build and that was causing traces not work in production.

Took a few hours of debugging!