Add WSGI helper methods

Add helper methods to make it easy to start a prometheus server as WSGI
app. This makes it easier to integrate with other WSGI applications like
flask or to start exposing metrics in your WSGI server of choice.

Author: rmohr

README.md

@@ -247,6 +247,32 @@ reactor.listenTCP(8000, factory)
 reactor.run()
 ```
 
+#### WSGI
+
+To use Prometheus with [WSGI](http://wsgi.readthedocs.org/en/latest/), there is
+`make_wsgi_app` which creates a WSGI application.
+
+```python
+from prometheus_client import make_wsgi_app
+from wsgiref.simple_server import make_server
+
+app = make_wsgi_app()
+httpd = make_server('', 8000, app)
+httpd.serve_forever()
+```
+
+Such an application can be useful when integrating Prometheus metrics with WSGI
+apps.
+
+The method `start_wsgi_server` can be used to serve the metrics through the
+WSGI reference implementation in a new thread.
+
+```python
+from prometheus_client import start_wsgi_server
+
+start_wsgi_server(8000)
+```
+
 ### Node exporter textfile collector
 
 The [textfile collector](https://github.com/prometheus/node_exporter#textfile-collector)

prometheus_client/__init__.py

@@ -19,7 +19,9 @@
 CONTENT_TYPE_LATEST = exposition.CONTENT_TYPE_LATEST
 generate_latest = exposition.generate_latest
 MetricsHandler = exposition.MetricsHandler
+make_wsgi_app = exposition.make_wsgi_app
 start_http_server = exposition.start_http_server
+start_wsgi_server = exposition.start_wsgi_server
 write_to_textfile = exposition.write_to_textfile
 push_to_gateway = exposition.push_to_gateway
 pushadd_to_gateway = exposition.pushadd_to_gateway

prometheus_client/exposition.py

@@ -7,6 +7,7 @@
 import time
 import threading
 from contextlib import closing
+from wsgiref.simple_server import make_server
 
 from . import core
 try:
@@ -23,10 +24,31 @@
     from urllib.parse import quote_plus
 
 
-CONTENT_TYPE_LATEST = 'text/plain; version=0.0.4; charset=utf-8'
+CONTENT_TYPE_LATEST = str('text/plain; version=0.0.4; charset=utf-8')
 '''Content type of the latest text format'''
 
 
+def make_wsgi_app():
+    '''Create a WSGI app which serves the metrics from the registry.'''
+    def prometheus_app(environ, start_response):
+        status = str('200 OK')
+        headers = [(str('Content-type'), CONTENT_TYPE_LATEST)]
+        start_response(status, headers)
+        return [generate_latest(core.REGISTRY)]
+    return prometheus_app
+
+
+def start_wsgi_server(port, addr=''):
+    """Starts a WSGI server for prometheus metrics as a daemon thread."""
+    class PrometheusMetricsServer(threading.Thread):
+        def run(self):
+            httpd = make_server(addr, port, make_wsgi_app())
+            httpd.serve_forever()
+    t = PrometheusMetricsServer()
+    t.daemon = True
+    t.start()
+
+
 def generate_latest(registry=core.REGISTRY):
     '''Returns the metrics from the registry in latest text format as a string.'''
     output = []