Include SCRIPT_NAME when determining path

When determining the path for a request, `Rack::Request` prefixes the
`SCRIPT_NAME`, [as seen here][1]. This makes sure the relative path is
included. At work, we use [Iodine](https://github.com/boazsegev/iodine) and
Turbolinks, and without this patch most of our requests paths in Promethus are
`""`.  This patch fixes that.

[1]: https://github.com/rack/rack/blob/294fd239a71aab805877790f0a92ee3c72e67d79/lib/rack/request.rb#L512

Signed-off-by: Ian Ker-Seymer <i.kerseymer@gmail.com>

lib/prometheus/middleware/collector.rb

@@ -67,15 +67,17 @@ def trace(env)
       end
 
       def record(env, code, duration)
+        path = env[Rack::SCRIPT_NAME] + env[Rack::PATH_INFO]
+
         counter_labels = {
           code:   code,
           method: env['REQUEST_METHOD'].downcase,
-          path:   strip_ids_from_path(env['PATH_INFO']),
+          path:   strip_ids_from_path(path),
         }
 
         duration_labels = {
           method: env['REQUEST_METHOD'].downcase,
-          path:   strip_ids_from_path(env['PATH_INFO']),
+          path:   strip_ids_from_path(path),
         }
 
         @requests.increment(labels: counter_labels)