Instrument Grape Endpoint with Metrics::RackMiddleware

Generating the following tags

Grape#GET /projects/:id/archive

from Grape::Route objects like

{ :path => /:version/projects/:id/archive(.:format)
  :version => “v3”,
  :method => “GET” }

Use an instance variable to cache raw_path transformations.
This variable is only going to growth to the number of 
endpoints of the API, not with exact different requests

We can store this cache as an instance variable because 
middleware are initialised only once