Add blacklisting for wsgi apps

[lzchen]

Just like in Opencensus, users should be able to configure paths and hosts that they do not want to be traced.

Auto-instrumented apps would probably not have blacklisted paths/hosts.

[codeboten]

This would be super useful. Would like to see it renamed excluded list or filtered list.

[lzchen]

Currently, there are two ways of instrumenting a Flask application, through wsgi OpenTelemetryMiddleware and ext-flask FlaskInstrumenter. I believe the latter is intended to be just used for auto-instrumentation. We should have only one way of recommending users to instrument their Flask apps (manual). I believe @ocelotl is taking a look at this and whether we can leverage some of the existing wsgi logic for instruementation.

Until then, configuration for wsgi/flask apps is difficult to do, so might want to hold off on this until then.

[lzchen]

Continuation of [#353]