Fix duplicated prometheus messages

- Issue 34887 details prometheus metrics being duplicated once
  with one line having a 0 count and the other having a valid count

- Update lib/gitlab/metrics/requests_rack_middleware.rb to add
  .to_i to the status value.

- Add changelog for
  https://gitlab.com/gitlab-org/gitlab/merge_requests/19327

changelogs/unreleased/34887-fix-prom-duplicate-metrics.yml

+---
+title: Fix Prometheus duplicate metrics
+merge_request: 19327
+author:
+type: fixed

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 })
+            http_request_duration_seconds.get({ method: method, status: status.to_i })
           end
         end
       end

spec/lib/gitlab/metrics/requests_rack_middleware_spec.rb

@@ -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 }
+            expected_labels << { method: method, status: status.to_i }
           end
         end