Add eventlogging infrastructure

- Introduce the EventLog class from BinderHub for emitting
  structured event data
- Instrument server starts and stops to emit events
- Defaults to not saving any events anywhere

jupyterhub/handlers/base.py

@@ -156,6 +156,10 @@ class BaseHandler(RequestHandler):
     def oauth_provider(self):
         return self.settings['oauth_provider']
 
+    @property
+    def event_log(self):
+        return self.settings['event_log']
+
     def finish(self, *args, **kwargs):
         """Roll back any uncommitted transactions from the handler."""
         if self.db.dirty:
@@ -846,6 +850,11 @@ class BaseHandler(RequestHandler):
             SERVER_SPAWN_DURATION_SECONDS.labels(
                 status=ServerSpawnStatus.success
             ).observe(time.perf_counter() - spawn_start_time)
+            self.event_log.emit('hub.jupyter.org/server-action', 1, {
+                    'action': 'start',
+                    'username': user.name,
+                    'servername': server_name
+            })
             proxy_add_start_time = time.perf_counter()
             spawner._proxy_pending = True
             try:
@@ -1026,6 +1035,11 @@ class BaseHandler(RequestHandler):
                 SERVER_STOP_DURATION_SECONDS.labels(
                     status=ServerStopStatus.success
                 ).observe(toc - tic)
+                self.event_log.emit('hub.jupyter.org/server-action', 1, {
+                        'action': 'stop',
+                        'username': user.name,
+                        'servername': server_name
+                })
             except:
                 SERVER_STOP_DURATION_SECONDS.labels(
                     status=ServerStopStatus.failure