Sets up a filter to determine the log level for a given request.
For each HTTP request this will invoke request_filter to determine how much (if any) of
that request to log. (If you do not set a request filter, this will just always log requests at the
level passed to Logger.)
| this |
a Logger |
| request_filter |
the callback for request debugging |
| destroy |
a DestroyNotify to free |
| filter_data |
data to pass to the callback |