Add support for tornado.

[Resolves #114]

Signed-off-by: Joshua Carp <jm.carp@gmail.com>

README.md

@@ -280,6 +280,29 @@ reactor.listenTCP(8000, factory)
 reactor.run()
 ```
 
+#### Tornado
+
+To use prometheus with [tornado](https://www.tornadoweb.org/), there is `MetricsHandler` which exposes metrics as a tornado handler.
+
+```python
+from prometheus_client.tornado import MetricsHandler
+
+import tornado.ioloop
+import tornado.web
+
+def make_app():
+    return tornado.web.Application([
+        (r"/", MainHandler),
+    ])
+
+if __name__ == "__main__":
+    app = tornado.web.Application([
+        (r'/metrics', MetricsHandler)
+    ])
+    app.listen(8888)
+    tornado.ioloop.IOLoop.current().start()
+```
+
 #### WSGI
 
 To use Prometheus with [WSGI](http://wsgi.readthedocs.org/en/latest/), there is

prometheus_client/tornado/__init__.py

@@ -0,0 +1,3 @@
+from ._exposition import MetricsHandler
+
+__all__ = ['MetricsHandler']

prometheus_client/tornado/_exposition.py

@@ -0,0 +1,17 @@
+from __future__ import absolute_import, unicode_literals
+from .. import REGISTRY, exposition
+
+import tornado.web
+
+
+class MetricsHandler(tornado.web.RequestHandler):
+    """
+    Tornado ``Handler`` that serves prometheus metrics.
+    """
+    def initialize(self, registry=REGISTRY):
+        self.registry = registry
+
+    def get(self):
+        encoder, content_type = exposition.choose_encoder(self.request.headers.get('Accept'))
+        self.set_header('Content-Type', content_type)
+        self.write(encoder(self.registry))

setup.py

@@ -17,9 +17,11 @@
         'prometheus_client.bridge',
         'prometheus_client.openmetrics',
         'prometheus_client.twisted',
+        'prometheus_client.tornado',
     ],
     extras_require={
         'twisted': ['twisted'],
+        'tornado': ['tornado'],
     },
     test_suite="tests",
     classifiers=[

tests/test_tornado.py

@@ -0,0 +1,44 @@
+from __future__ import absolute_import, unicode_literals
+
+import sys
+
+if sys.version_info < (2, 7):
+    from unittest2 import skipUnless
+else:
+    from unittest import skipUnless
+
+from prometheus_client import Counter
+from prometheus_client import CollectorRegistry, generate_latest
+
+try:
+    from prometheus_client.tornado import MetricsHandler
+
+    import tornado.ioloop
+    import tornado.web
+    import tornado.testing
+    TestCase = tornado.testing.AsyncHTTPTestCase
+    HAVE_TORNADO = True
+except ImportError:
+    from unittest import TestCase
+    HAVE_TORNADO = False
+
+
+class MetricsResourceTest(TestCase):
+    def get_app(self):
+        self.registry = CollectorRegistry()
+        return tornado.web.Application([
+            (r'/metrics', MetricsHandler, {'registry': self.registry}),
+        ])
+
+    @skipUnless(HAVE_TORNADO, "Don't have tornado installed.")
+    def test_reports_metrics(self):
+        """
+        ``MetricsHandler`` serves the metrics from the provided registry.
+        """
+        c = Counter('cc', 'A counter', registry=self.registry)
+        c.inc()
+
+        resp = self.fetch('/metrics')
+
+        self.assertEqual(resp.code, 200)
+        self.assertEqual(resp.body, generate_latest(self.registry))

tox.ini

@@ -30,6 +30,7 @@ deps =
 deps =
     {[base]deps}
     {py27,py34,py35,py36,pypy}: twisted
+    {py27,py34,py35,py36,pypy}: tornado
 commands = coverage run --parallel -m pytest {posargs}