opentelmetry-example-app should be moved to contrib repo

[mariojonke]

The opentelemetry_example_app in docs/examples has a setup.cfg with dependencies to opentelemetry-instrumentation-{requets,flask}. These dependencies refer to an already released version 0.15.b0 (i think this should be 0.15b0 instead?).
Since there are tox tests for opentelemtry-example-app they might fail if api and/or sdk in dev version get incompatible with the releassed instrumentation. Also upgrading the dependnecies might be overlooked when releasing a new version since the version string differs.

What is the expected behavior?
Imo opentelemetry_example_app should be moved to the contrib repo since it is mostly requests and flask samples?

[NathanielRN]

+1 to this, had an issue with linting it today which made me think that this should really be a Contrib repo item. See #1357

[lzchen]

I'm wondering do we even need this example app anymore? I'm not sure where we would even put it if we move it to the contrib repo.

@codeboten Thoughts?