Add basic instrumentation possibilities to pretix (#271)

* add basic instrumentation possibilities to pretix

* make tabs to spaces

* apply flake8

* implement upstreams suggestions, round 1

* adjust naming of redis-connection

* address noredis

* add view for metrics

* implement HTTP basic auth in front of metrics-endpoint

* rename labelset

* make flake8-clean

* implement upstreams suggestions, round 2

* correct minor slipups

* fix missing return

* let isort add an empty line

* implement test for counter

* implement upstream suggestions, round 3

* correct typo

* implement first test for view

* finish view-test

* fix deprecated keyword

* implement upstream-suggestions, round 4

* implement test for gauge

* test exceptions as well

* add db-decorator

doc/admin/config.rst

@@ -174,6 +174,20 @@ Example::
     .. WARNING:: Never set this to ``True`` in production!
 
 
+Metrics
+-------
+
+If you want to fetch internally collected prometheus-style metrics you need to configure the credentials for the
+metrics endpoint and enable it::
+
+    [metrics]
+    enabled=true
+    user=your_user
+    passphrase=mysupersecretpassphrase
+
+Currently, metrics-collection requires a redis server to be available.
+
+
 Memcached
 ---------