Use strings for request status label

Prometheus metric labels are always strings. This will avoid passing
invalid ints to the metric library in case it rejects non-string label
value input.

Related to https://gitlab.com/gitlab-org/gitlab/issues/34887

lib/gitlab/metrics/requests_rack_middleware.rb

@@ -35,7 +35,7 @@ module Gitlab
       def self.initialize_http_request_duration_seconds
         HTTP_METHODS.each do |method, statuses|
           statuses.each do |status|
-            http_request_duration_seconds.get({ method: method, status: status.to_i })
+            http_request_duration_seconds.get({ method: method, status: status.to_s })
           end
         end
       end
@@ -49,7 +49,7 @@ module Gitlab
           status, headers, body = @app.call(env)
 
           elapsed = Time.now.to_f - started
-          RequestsRackMiddleware.http_request_duration_seconds.observe({ method: method, status: status }, elapsed)
+          RequestsRackMiddleware.http_request_duration_seconds.observe({ method: method, status: status.to_s }, elapsed)
 
           [status, headers, body]
         rescue

spec/lib/gitlab/metrics/requests_rack_middleware_spec.rb

@@ -31,7 +31,7 @@ describe Gitlab::Metrics::RequestsRackMiddleware do
       end
 
       it 'measures execution time' do
-        expect(described_class).to receive_message_chain(:http_request_duration_seconds, :observe).with({ status: 200, method: 'get' }, a_positive_execution_time)
+        expect(described_class).to receive_message_chain(:http_request_duration_seconds, :observe).with({ status: '200', method: 'get' }, a_positive_execution_time)
 
         Timecop.scale(3600) { subject.call(env) }
       end
@@ -69,7 +69,7 @@ describe Gitlab::Metrics::RequestsRackMiddleware do
         expected_labels = []
         described_class::HTTP_METHODS.each do |method, statuses|
           statuses.each do |status|
-            expected_labels << { method: method, status: status.to_i }
+            expected_labels << { method: method, status: status.to_s }
           end
         end