handle unoconv renderer's delayed file_required

 * unoconv's renderer proxies its file_required property to its
   subrenderer's file_required.  The subrenderer isn't constructed until
   the end of __init__.  Special case file_required, since the base
   implementation works for all the other renderers.

mfr/core/extension.py

@@ -50,12 +50,18 @@ def __init__(self, metadata, file_path, url, assets_url, export_url):
             'url': self.url,
             'export_url': self.export_url,
             'file_path': self.file_path,
-            'file_required': self.file_required,
             'cache_result': self.cache_result,
             # 'error': 'error_t',
             # 'elapsed': 'elpased_t',
         })
 
+        # unoconv gets file_required from its subrenderer, which is constructed at the end of
+        # __init__.
+        try:
+            self.renderer_metrics.add('file_required', self.file_required)
+        except AttributeError:
+            pass
+
     @abc.abstractmethod
     def render(self):
         pass

mfr/extensions/unoconv/render.py

@@ -34,6 +34,8 @@ def __init__(self, metadata, file_path, url, assets_url, export_url):
             export_url
         )
 
+        # can't call file_required until renderer is built
+        self.renderer_metrics.add('file_required', self.file_required)
         self.metrics.merge({
             'map': {
                 'renderer': self.map['renderer'],