Takes into account the SCRIPT_NAME in path

[woto]

Hi! Not sure that this is correct, but:

1. We have lost the path using mountable engines (twirp, grape)
2. It looks like the RFC says that this is correct: SCRIPT_NAME, PATH_NFO
3. https://github.com/rack/rack/blob/master/lib/rack/handler/webrick.rb#L92

Sorry without tests for now. Want to hear your opinion.

[coveralls]

Coverage remained the same at 100.0% when pulling 4e63de0 on severgroup-tt:master into 872a8eb on prometheus:master.

[dmagliola]

Hi @woto,

Thank you for your PR.
This is the same issue from PR #197

Our concern with this is that it feels like it's a backwards incompatible change, and our recommendation at the time is to use this built-in collector as an "example" that you are encouraged to use as a base to make your own if it isn't fit for purpose.

I'd like to close the PR and move the conversation to #197 if that's ok, so we can have it in one place.
Thanks!

[woto]

Ok, fine thank you

[dmagliola]

Ok, we've finally had time to look into this enough to convince ourselves this is definitely the right thing to do.
I wanted to add tests to this before merging, but I wasn't able to in this same branch, so I made a new PR #206
I've added you as a co-author on the commit, to give credit for the code and idea, hope that's ok!

I'm going to merge #206 son, and cut a new major version soon that fixes this.

Thank you for bringing this to our attention and for the PR!