Get controller labels from controller, not params

[richardTowers]

I've got an open PR to do this for the default default in prometheus_exporter itself:

discourse/prometheus_exporter/pull/293

But it's unclear whether the maintainers will accept that, and if so how long that will take.

In the meantime, we've got quite a bit of noise in our metrics because it's currently possible for an HTTP request to overwrite the action / controller labels by passing them in as request parameters like this:

curl -v http://127.0.0.1:3000/ --data 'controller=test'

This commit extends the default middleware so we have more sensible defaults for both Rails and Sinatra apps. I couldn't work out any good labels for sinatra apps that wouldn't be potentially high cardinality, so those are just blank.

[sihugh]

Feels a bit surprising to get something from env and then access object properties directly rather than accessing a hash/array/string. I guess it works though?

[richardTowers]

Feels a bit surprising to get something from env and then access object properties directly rather than accessing a hash/array/string. I guess it works though?

Yeah, it's a bit weird, but some of the values on the env hashes are full on object instances. action_controller.instance is the current controller's instance.

I've tested it locally (as this isn't covered by any tests in the gem 😬), and it does work in frontend.