make flask request attributes avaiable for sampler

[srikanthccv]

          Hi again, could we add another issue for [opentelemetry-instrumentation-flask](https://github.com/open-telemetry/opentelemetry-python-contrib/tree/main/instrumentation/opentelemetry-instrumentation-flask)? I'd like to take that one too. The change is probably similar to the [recent Django update](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/1730/files).

Originally posted by @tammy-baylis-swi in #936 (comment)

[srikanthccv]

Please leave a comment so that I can assign it to you.

[tammy-baylis-swi]

Thanks @srikanthccv ! I'd like to take this one.