Flask: Path Templates Support

[pglombardo]

This PR adds support for Path Templates under Flask.

Flask routes such as:

@app.route("/users/<username>/sayhello")
def username_hello(username):
    return "<center><h1>🐍 Hello %s! 🦄</h1></center>" % username

will now show as grouped endpoints in Instana Analyze:

...and when drilling down:

Read more about endpoint grouping here:
https://docs.instana.io/products/application_service_management/#endpoint-grouping---path-templates