Check for json log formatting in log_request

This gets us closer to logging parts of the request
as separate json fields but not all the way there.

For example:

```json
{"written_at": "2020-10-07T21:52:16.975Z", "written_ts": 1602107536975497000, "msg": "{'status': 404, 'method': 'GET', 'ip': '127.0.0.1', 'uri': '/nbextensions/widgets/notebook/js/extension.js', 'request_time': 1.9807815551757812, 'referer': 'http://localhost:8888/notebooks/test.ipynb'}", "type": "log", "logger": "NotebookApp", "thread": "MainThread", "level": "WARNING", "module": "log", "line_no": 54}
```

notebook/log.py

@@ -9,6 +9,10 @@ import json
 from tornado.log import access_log
 from .prometheus.log_functions import prometheus_log_method
 
+from .utils import enable_json_logs
+
+_enable_json_logs = enable_json_logs()
+
 
 def log_request(handler):
     """log a bit more information about each request than tornado's default
@@ -46,5 +50,11 @@ def log_request(handler):
     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 _enable_json_logs:
+        # FIXME: this still logs the msg as a serialized json string,
+        # presumably because it's using tornado's access_log rather than
+        # the logger setup in notebook app with _log_formatter_cls.
+        log_method(ns)
+    else:
+        log_method(msg.format(**ns))
     prometheus_log_method(handler)

notebook/notebookapp.py

@@ -113,6 +113,7 @@ from notebook._sysinfo import get_sys_info
 from ._tz import utcnow, utcfromtimestamp
 from .utils import (
     check_pid,
+    enable_json_logs,
     pathname2url,
     run_sync,
     unix_socket_in_use,
@@ -139,17 +140,17 @@ except ImportError:
     terminado_available = False
 
 # Tolerate missing json_logging package.
-enable_json_logs = os.getenv('ENABLE_JSON_LOGGING', 'false').lower() == 'true'
+_enable_json_logs = enable_json_logs()
 try:
     import json_logging
 except ImportError:
     # If configured for json logs and we can't do it, log a hint.
-    if enable_json_logs:
+    if _enable_json_logs:
         logging.getLogger(__name__).exception(
             'Unable to use json logging due to missing packages. '
             'Run "pip install notebook[json-logging]" to fix.'
         )
-    enable_json_logs = False
+    _enable_json_logs = False
 
 #-----------------------------------------------------------------------------
 # Module globals
@@ -717,7 +718,7 @@ class NotebookApp(JupyterApp):
 
     # Unless there is a way to re-initialize the log formatter (like with _log_format_changed?)
     # we need to load the json logging formatter early here otherwise traitlets complains.
-    _log_formatter_cls = json_logging.JSONLogFormatter if enable_json_logs else LogFormatter
+    _log_formatter_cls = json_logging.JSONLogFormatter if _enable_json_logs else LogFormatter
 
     @default('log_level')
     def _default_log_level(self):

notebook/utils.py

@@ -401,3 +401,7 @@ def unix_socket_in_use(socket_path):
         return True
     finally:
         sock.close()
+
+
+def enable_json_logs():
+    return os.getenv('ENABLE_JSON_LOGGING', 'false').lower() == 'true'