Expose set_metric_tag consistently.

Just a no-op on the statsd implementation.

docs/api.rst

@@ -21,6 +21,7 @@ implements the same interface:
          self.record_timing(self.request.request_time(), 'request', 'lookup')
 
    .. method:: increase_counter(*path, amount=1)
+      :noindex:
 
       :param path: counter path to increment
       :keyword int amount: value to increase the counter by
@@ -30,6 +31,7 @@ implements the same interface:
          self.increase_counter('db', 'query', 'foo')
 
    .. method:: execution_timer(*path)
+      :noindex:
 
       :param path: timing path to record
 
@@ -41,6 +43,16 @@ implements the same interface:
          with self.execution_timer('db', 'query', 'foo'):
              rows = yield self.session.query('SELECT * FROM foo')
 
+   .. method:: set_metric_tag(tag, value)
+      :noindex:
+
+      :param str tag: the tag to set
+      :param str value: the value to assign to the tag
+
+      This method stores a tag and value pair to be reported with
+      metrics.  It is only implemented on back-ends that support
+      tagging metrics (e.g., :class:`sprockets.mixins.metrics.InfluxDBMixin`)
+
 
 Statsd Implementation
 ---------------------

examples/statsd.py

@@ -14,6 +14,16 @@ class SimpleHandler(metrics.StatsdMixin, web.RequestHandler):
 
     """
 
+    @gen.coroutine
+    def prepare(self):
+        maybe_future = super(SimpleHandler, self).prepare()
+        if gen.is_future(maybe_future):
+            yield maybe_future
+
+        if 'Correlation-ID' in self.request.headers:
+            self.set_metric_tag('correlation_id',
+                                self.request.headers['Correlation-ID'])
+
     @gen.coroutine
     def get(self):
         yield gen.sleep(0.25)

sprockets/mixins/metrics/statsd.py

@@ -39,6 +39,9 @@ class StatsdMixin(object):
         settings.setdefault('port', '8125')
         self.__status_code = None
 
+    def set_metric_tag(self, tag, value):
+        """Ignored for statsd since it does not support tagging."""
+
     def set_status(self, status_code, reason=None):
         # Extended to track status code to avoid referencing the
         # _status internal variable

tests.py

@@ -107,6 +107,11 @@ class StatsdMethodTimingTests(testing.AsyncHTTPTestCase):
         for path, value, stat_type in self.statsd.find_metrics(prefix, 'ms'):
             assert_between(250.0, float(value), 300.0)
 
+    def test_that_add_metric_tag_is_ignored(self):
+        response = self.fetch('/',
+                              headers={'Correlation-ID': 'does not matter'})
+        self.assertEqual(response.code, 204)
+
 
 class InfluxDbTests(testing.AsyncHTTPTestCase):