Expose function to return trace header

[psteinroe]

We are using QStash and would like to setup distributed tracing across jobs. Unfortunately, QStash adds a prefix to the custom headers. Afaik, this library extracts potential parent traces from the standard header. Is it possible to make the name of the header configurable? Or even simpler, add a "extract" function that we can run before the headers are accessed and that returns the new "header" object?