Support for dynamic headers in Zipkin exporter

[sfishel-splunk]

Is your feature request related to a problem? Please describe.

I have an application where the auth token I'm using for the Zipkin exporter endpoint headers can change over the lifetime of the page. The exporter API only supports setting headers at instantiation time, so I have no way to update the token.

Describe the solution you'd like

I'm open to any solution, but what I had in mind was support for a function as the header property in the exporter configuration, which would be called as each export request is sent.

Describe alternatives you've considered

I have attempted to accomplish this by sub-classing the Zipkin exporter, but I can't do it without referencing private methods.

Additional context

I would be up for submitting a pull request if you're open to this idea!