Include SCRIPT_NAME when determining path in Collector

When determining the path for a request, `Rack::Request` prefixes the
`SCRIPT_NAME`, [as seen here][1].

This is a problem with our current code when using mountable engines,
where the engine part of the path gets lost.

This patch fixes that to include `SCRIPT_NAME` as part of the path.

NOTE: This is not backwards compatible. Labels will change in existing
metrics. We will cut a new major version once we ship this.

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

lib/prometheus/middleware/collector.rb

@@ -67,15 +67,17 @@ def trace(env)
       end
 
       def record(env, code, duration)
+        path = [env["SCRIPT_NAME"], env['PATH_INFO']].join
+
         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)

spec/prometheus/middleware/collector_spec.rb

@@ -55,6 +55,18 @@
     expect(registry.get(metric).get(labels: labels)).to include("0.1" => 0, "0.25" => 1)
   end
 
+  it 'includes SCRIPT_NAME in the path if provided' do
+    metric = :http_server_requests_total
+
+    get '/foo'
+    expect(registry.get(metric).values.keys.last[:path]).to eql("/foo")
+
+    env('SCRIPT_NAME', '/engine')
+    get '/foo'
+    env('SCRIPT_NAME', nil)
+    expect(registry.get(metric).values.keys.last[:path]).to eql("/engine/foo")
+  end
+
   it 'normalizes paths containing numeric IDs by default' do
     expect(Benchmark).to receive(:realtime).and_yield.and_return(0.3)