Add exemplar support to all WSGI based instrumentations #2158
Description
Describe the solution you'd like
I would like that recorded spans from the WSGI instrumentation (in particular the Flask one) also provide exemplars. Currently there is no link between the spans and the related metrics.
Describe alternatives you've considered
Up to now, I used custom histogram and custom spans (in my own request handler). But this is redundant with the more integrated instrumentation setup.
Additional context
According to the OTel documentation, any metric published inside an active span get linked as an exemplar. However, the latency of the handler seems to be reported after the handler-span has ended. So no link is recorded between the span and the metric, while both are clearly related.