Use Alert#starts_at_raw for gitlab_fingerprint

Fixes `gitlab_fingerprint` by using `starts_at_row`

lib/gitlab/alerting/alert.rb

@@ -121,9 +121,9 @@ module Gitlab
 
       def plain_gitlab_fingerprint
         if gitlab_managed?
-          [metric_id, starts_at].join('/')
+          [metric_id, starts_at_raw].join('/')
         else # self managed
-          [starts_at, title, full_query].join('/')
+          [starts_at_raw, title, full_query].join('/')
         end
       end
 

spec/lib/gitlab/alerting/alert_spec.rb

@@ -253,7 +253,7 @@ describe Gitlab::Alerting::Alert do
       include_context 'gitlab alert'
 
       it 'returns a fingerprint' do
-        plain_fingerprint = [alert.metric_id, alert.starts_at].join('/')
+        plain_fingerprint = [alert.metric_id, alert.starts_at_raw].join('/')
 
         is_expected.to eq(Digest::SHA1.hexdigest(plain_fingerprint))
       end
@@ -263,7 +263,7 @@ describe Gitlab::Alerting::Alert do
       include_context 'full query'
 
       it 'returns a fingerprint' do
-        plain_fingerprint = [alert.starts_at, alert.title, alert.full_query].join('/')
+        plain_fingerprint = [alert.starts_at_raw, alert.title, alert.full_query].join('/')
 
         is_expected.to eq(Digest::SHA1.hexdigest(plain_fingerprint))
       end