Log request properties separately when log_json=True

This updates the log_request method to get the request
properties logged separately if `log_json=True`. This
ends up looking like:

```
{"written_at": "2020-10-09T20:59:52.044Z", "written_ts": 1602277192044732000, "msg": "", "type": "log", "logger": "NotebookApp", "thread": "MainThread", "level": "WARNING", "module": "log", "line_no": 50, "status": 404, "method": "GET", "ip": "127.0.0.1", "uri": "/api/kernels/a6dd447b-282a-4e1c-ab65-eb340ad12965/channels?session_id=472907c27ed94e2cbfbae86501f7d159", "request_time": 8.63, "referer": "None"}
```

notebook/log.py

@@ -10,7 +10,7 @@ from tornado.log import access_log
 from .prometheus.log_functions import prometheus_log_method
 
 
-def log_request(handler, log=access_log):
+def log_request(handler, log=access_log, log_json=False):
     """log a bit more information about each request than tornado's default
     
     - move static file get success to debug-level (reduces noise)
@@ -36,9 +36,9 @@ def log_request(handler, log=access_log):
         method=request.method,
         ip=request.remote_ip,
         uri=request.uri,
-        request_time=request_time,
+        request_time=float('%.2f' % request_time),
     )
-    msg = "{status} {method} {uri} ({ip}) {request_time:.2f}ms"
+    msg = "{status} {method} {uri} ({ip}) {request_time:f}ms"
     if status >= 400:
         # log bad referers
         ns['referer'] = request.headers.get('Referer', 'None')
@@ -46,5 +46,8 @@ def log_request(handler, log=access_log):
     if status >= 500 and status != 502:
         # log all headers if it caused an error
         log_method(json.dumps(dict(request.headers), indent=2))
-    log_method(msg.format(**ns))
+    if log_json:
+        log_method("", extra=dict(props=ns))
+    else:
+        log_method(msg.format(**ns))
     prometheus_log_method(handler)

notebook/notebookapp.py

@@ -252,7 +252,8 @@ class NotebookWebApplication(web.Application):
             root_dir = '~' + root_dir[len(home):]
 
         # Use the NotebookApp logger and its formatting for tornado request logging.
-        log_function = functools.partial(log_request, log=log)
+        log_function = functools.partial(
+            log_request, log=log, log_json=jupyter_app.log_json)
         settings = dict(
             # basics
             log_function=log_function,