Render format csv

[Dieterbe]

most of the code is copy pasted from the json and pickle format (e.g. all the benchmarking boilerplate), so this should be easy to review (just focus on the csv specific bits)

tested with docker-dev. seems to work fine.

$ curl 'http://localhost:6060/render?target=metrictank.stats.docker-env.default.tank.metrics_active.gauge32&from=-10s&format=csv' 
metrictank.stats.docker-env.default.tank.metrics_active.gauge32,2021-05-04 12:46:54,614
metrictank.stats.docker-env.default.tank.metrics_active.gauge32,2021-05-04 12:46:55,614
metrictank.stats.docker-env.default.tank.metrics_active.gauge32,2021-05-04 12:46:56,614
metrictank.stats.docker-env.default.tank.metrics_active.gauge32,2021-05-04 12:46:57,614
metrictank.stats.docker-env.default.tank.metrics_active.gauge32,2021-05-04 12:46:58,614
metrictank.stats.docker-env.default.tank.metrics_active.gauge32,2021-05-04 12:46:59,614
metrictank.stats.docker-env.default.tank.metrics_active.gauge32,2021-05-04 12:47:00,614
metrictank.stats.docker-env.default.tank.metrics_active.gauge32,2021-05-04 12:47:01,614
metrictank.stats.docker-env.default.tank.metrics_active.gauge32,2021-05-04 12:47:02,614
metrictank.stats.docker-env.default.tank.metrics_active.gauge32,2021-05-04 12:47:03,614
$ curl 'http://localhost:6060/render?target=metrictank.stats.docker-env.default.tank.metrics_active.gauge32&from=-10s&format=json'
[{"target":"metrictank.stats.docker-env.default.tank.metrics_active.gauge32","tags":{"name":"metrictank.stats.docker-env.default.tank.metrics_active.gauge32"},"datapoints":[[614,1620132414],[614,1620132415],[614,1620132416],[614,1620132417],[614,1620132418],[614,1620132419],[614,1620132420],[614,1620132421],[614,1620132422],[614,1620132423]]}]

[robert-milan]

LGTM, just the one comment.