Describe your environment.
Python 3.7, Flask 1.1, OpenCensus 0.7.3

Steps to reproduce.

1. Run opencensus with the Flask middleware enabled with any Flask app
2. Hit a URL that does not map to a rule (ie. /foo)

What is the expected behavior?
Don't expect exceptions thrown for 404 routes

What is the actual behavior?
Every 404 route is generating exceptions

Additional context.

The problem is caused by the following PR: #759

It assumes that the url_rule is not None -- if you hit any non-existent URL (ie. a 404-triggering URL), then request.url_rule will be None and code will raise an AttributeError