Add feature category to API latency histogram

This populates the `feature_category` label on the
`gitlab_transaction_duration_seconds` histogram.

This histogram already contains this information for regular
controller requests.

The feature category per endpoint is always the same, so this doesn't
really increase cardinality.

lib/gitlab/metrics/web_transaction.rb

@@ -66,9 +66,10 @@ module Gitlab
         if route
           path = endpoint_paths_cache[route.request_method][route.path]
 
-          # Feature categories will be added for grape endpoints in
-          # https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/462
-          { controller: 'Grape', action: "#{route.request_method} #{path}", feature_category: '' }
+          grape_class = endpoint.options[:for]
+          feature_category = grape_class.try(:feature_category_for_app, endpoint).to_s
+
+          { controller: 'Grape', action: "#{route.request_method} #{path}", feature_category: feature_category }
         end
       end
 

spec/lib/gitlab/metrics/web_transaction_spec.rb

@@ -80,13 +80,15 @@ RSpec.describe Gitlab::Metrics::WebTransaction do
     context 'when request goes to Grape endpoint' do
       before do
         route = double(:route, request_method: 'GET', path: '/:version/projects/:id/archive(.:format)')
-        endpoint = double(:endpoint, route: route)
+        endpoint = double(:endpoint, route: route,
+                          options: { for: API::Projects, path: [":id/archive"] },
+                          namespace: "/projects")
 
         env['api.endpoint'] = endpoint
       end
 
       it 'provides labels with the method and path of the route in the grape endpoint' do
-        expect(transaction.labels).to eq({ controller: 'Grape', action: 'GET /projects/:id/archive', feature_category: '' })
+        expect(transaction.labels).to eq({ controller: 'Grape', action: 'GET /projects/:id/archive', feature_category: 'projects' })
       end
 
       it 'contains only the labels defined for transactions' do