InfluxDBMixin: Move call to super.__init__.

`tornado.web.RequestHandler.__init__` calls
`tornado.web.RequestHandler.initialize` to initialize the object.
Sub-classes should be able to call our methods to tag metrics in
their `initialize` implementations.  For this to work we need to
set our internal attributes *BEFORE* we call `super.__init__`.

docs/history.rst

@@ -3,6 +3,12 @@
 Release History
 ===============
 
+`Next Release`_
+---------------
+- Make it possible to call methods (e.g.,
+  :meth:`~sprockets.mixins.metrics.influxdb.InfluxDBMixin.set_metric_tag`)
+  during the Tornado request handler initialization phase.
+
 `2.0.0`_ (11-Mar-2016)
 ----------------------
 - Rework InfluxDB buffering to use a periodic callback instead of flushing

examples/influxdb.py

@@ -31,6 +31,10 @@ class SimpleHandler(influxdb.InfluxDBMixin, web.RequestHandler):
 
     """
 
+    def initialize(self):
+        super(SimpleHandler, self).initialize()
+        self.set_metric_tag('environment', 'testing')
+
     @gen.coroutine
     def prepare(self):
         maybe_future = super(SimpleHandler, self).prepare()

sprockets/mixins/metrics/influxdb.py

@@ -20,7 +20,6 @@ class InfluxDBMixin(object):
     """Mix this class in to record measurements to a InfluxDB server."""
 
     def __init__(self, application, request, **kwargs):
-        super(InfluxDBMixin, self).__init__(application, request, **kwargs)
         self.__metrics = []
         self.__tags = {
             'handler': '{}.{}'.format(self.__module__,
@@ -28,6 +27,11 @@ class InfluxDBMixin(object):
             'method': request.method,
         }
 
+        # Call to super().__init__() needs to be *AFTER* we create our
+        # properties since it calls initialize() which may want to call
+        # methods like ``set_metric_tag``
+        super(InfluxDBMixin, self).__init__(application, request, **kwargs)
+
     def set_metric_tag(self, tag, value):
         """
         Add a tag to the measurement key.