Allow specifying statsd host/port/prefix info

Currently only passes it through to CHP. This is needed
for the cases when JupyterHub spawns and maintains CHP.

jupyterhub/app.py

@@ -449,6 +449,20 @@ class JupyterHub(Application):
         """
     ).tag(config=True)
 
+    statsd_host = Unicode(
+        help="Host to send statds metrics to"
+    ).tag(config=True)
+
+    statsd_port = Int(
+        8125,
+        help="Port on which to send statsd metrics about the hub"
+    ).tag(config=True)
+
+    statsd_prefix = Unicode(
+        'jupyterhub',
+        help="Prefix to use for all metrics sent by jupyterhub to statsd"
+    )
+
     handlers = List()
 
     _log_formatter_cls = CoroutineLogFormatter
@@ -867,6 +881,12 @@ class JupyterHub(Application):
             cmd.extend(['--ssl-key', self.ssl_key])
         if self.ssl_cert:
             cmd.extend(['--ssl-cert', self.ssl_cert])
+        if self.statsd_host:
+            cmd.extend([
+                '--statsd-host', self.statsd_host,
+                '--statsd-port', self.statsd_port,
+                '--statsd-prefix', self.statsd_prefix + '.chp'
+            ])
         # Require SSL to be used or `--no-ssl` to confirm no SSL on
         if ' --ssl' not in ' '.join(cmd):
             if self.confirm_no_ssl: