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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _set_request_referer_metric(self, request):
 """ Add metric 'request_referer' for http referer. """

        if 'HTTP_REFERER' in request.META and request.META['HTTP_REFERER']:
            monitoring.set_custom_metric('request_referer', request.META['HTTP_REFERER'])