Fix feature_category label in http_requests_total metric

When the feature category header from the application was an empty
string, we'd set an empty label in Prometheus, rather than the default
of 'unknown'.

lib/gitlab/metrics/requests_rack_middleware.rb

@@ -63,7 +63,7 @@ module Gitlab
           if health_endpoint
             RequestsRackMiddleware.http_health_requests_total.increment(status: status, method: method)
           else
-            RequestsRackMiddleware.http_request_total.increment(status: status, method: method, feature_category: feature_category || FEATURE_CATEGORY_DEFAULT)
+            RequestsRackMiddleware.http_request_total.increment(status: status, method: method, feature_category: feature_category.presence || FEATURE_CATEGORY_DEFAULT)
           end
         end
       end

spec/lib/gitlab/metrics/requests_rack_middleware_spec.rb

@@ -113,6 +113,7 @@ RSpec.describe Gitlab::Metrics::RequestsRackMiddleware do
       end
     end
 
+    context 'feature category header' do
       context 'when a feature category header is present' do
         before do
           allow(app).to receive(:call).and_return([200, { described_class::FEATURE_CATEGORY_HEADER => 'issue_tracking' }, nil])
@@ -134,6 +135,19 @@ RSpec.describe Gitlab::Metrics::RequestsRackMiddleware do
         end
       end
 
+      context 'when the feature category header is an empty string' do
+        before do
+          allow(app).to receive(:call).and_return([200, { described_class::FEATURE_CATEGORY_HEADER => '' }, nil])
+        end
+
+        it 'sets the feature category to unknown' do
+          expect(described_class).to receive_message_chain(:http_request_total, :increment).with(method: 'get', status: 200, feature_category: 'unknown')
+
+          subject.call(env)
+        end
+      end
+    end
+
     describe '.initialize_http_request_duration_seconds' do
       it "sets labels" do
         expected_labels = []