Add file publisher entrypoint to publish events

Implementation to publish event using file publisher already exists.

Due to the missing entrypoint for file publisher, events couldn't
be written to a file.

Closes-Bug: 2033052

Change-Id: Iee69978121874263d6de67dbddd420e44b6bff4c
(cherry picked from commit 3f874056c3)

ceilometer/publisher/file.py

@@ -109,6 +109,7 @@ class FilePublisher(publisher.ConfigPublisherBase):
         if self.publisher_logger:
             for event in events:
                 if self.output_json:
-                    self.publisher_logger.info(json.dumps(event.as_dict()))
+                    self.publisher_logger.info(json.dumps(event.as_dict(),
+                                                          default=str))
                 else:
                     self.publisher_logger.info(event.as_dict())

setup.cfg

@@ -196,6 +196,7 @@ ceilometer.event.publisher =
     https = ceilometer.publisher.http:HttpPublisher
     gnocchi = ceilometer.publisher.gnocchi:GnocchiPublisher
     zaqar = ceilometer.publisher.zaqar:ZaqarPublisher
+    file = ceilometer.publisher.file:FilePublisher
 
 ceilometer.event.trait_plugin =
     split = ceilometer.event.trait_plugins:SplitterTraitPlugin