Add a /metrics endpoint for Prometheus Metrics

[Prometheus](https://prometheus.io/) provides a standard
metrics format that can be collected and used in many contexts.

- From the browser
  to drive 'current resource usage' displays, such
  as https://github.com/yuvipanda/nbresuse
- From a prometheus server
  to collect historical data for operational analysis and
  performance monitoring
  Example: https://grafana.mybinder.org/dashboard/db/1-overview?refresh=1m&orgId=1
  for mybinder.org metrics from JupyterHub and BinderHub,
  via prometheus server at https://prometheus.mybinder.org

The JupyterHub and BinderHub projects already expose Prometheus
metrics natively. Adding this to the Jupyter notebook server
allows us to instrument the code easily and in
a standard format that has lots of 3rd party tooling for it.

This commit does the following:

- Introduce the `prometheus_client` library as a dependency.
  This library has no dependencies of its own and is pure python.
- Add an authenticated `/metrics` endpoint to the server,
  which returns metrics in Prometheus Text Format
- Expose the default process metrics from `prometheus_client`,
  which include memory usage and CPU usage info (for just the
  notebook process)

Author: yuvipanda

notebook/base/handlers.py

@@ -28,6 +28,7 @@
 from jinja2 import TemplateNotFound
 from tornado import web, gen, escape, httputil
 from tornado.log import app_log
+import prometheus_client
 
 from notebook._sysinfo import get_sys_info
 
@@ -809,6 +810,16 @@ def get(self):
         url = sep.join([self._url, self.request.query])
         self.redirect(url, permanent=self._permanent)
 
+class PrometheusMetricsHandler(IPythonHandler):
+    """
+    Return prometheus metrics for this notebook server
+    """
+    @web.authenticated
+    def get(self):
+        self.set_header('Content-Type', prometheus_client.CONTENT_TYPE_LATEST)
+        self.write(prometheus_client.generate_latest(prometheus_client.REGISTRY))
+
+
 #-----------------------------------------------------------------------------
 # URL pattern fragments for re-use
 #-----------------------------------------------------------------------------
@@ -825,4 +836,5 @@ def get(self):
     (r".*/", TrailingSlashHandler),
     (r"api", APIVersionHandler),
     (r'/(robots\.txt|favicon\.ico)', web.StaticFileHandler),
+    (r'/metrics', PrometheusMetricsHandler)
 ]

setup.py

@@ -88,6 +88,7 @@
         'ipykernel', # bless IPython kernel for now
         'Send2Trash',
         'terminado>=0.8.1'
+        'prometheus_client'
     ],
     extras_require = {
         'test:python_version == "2.7"': ['mock'],