* 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
`"-- "` on a single line as a signature-delimiter is noticed by a lot of mail programs out there as an actual signature separator and the text below may be marked that way, e.g. is greyed out a little in Mozilla Thunderbird.
This change should leverage that behavior for e-mails sent by pretix.
Jonas Große Sundrup