Fix insights `period_field` parameter

This change casts the `period_field` parameter to Symbol so the
validation will not detect an error when a string `period_field` is
given.

ee/changelogs/unreleased/fix-insights-period-field.yml

+---
+title: Fix insights period_field parameter
+merge_request: 38404
+author:
+type: fixed

ee/lib/gitlab/insights/reducers/count_per_period_reducer.rb

@@ -45,8 +45,8 @@ module Gitlab
             raise InvalidPeriodError, "Invalid value for `period`: `#{period}`. Allowed values are #{VALID_PERIOD}!"
           end
 
-          unless VALID_PERIOD_FIELDS[issuable_type].include?(period_field)
-            raise InvalidPeriodFieldError, "Invalid value for `period_field`: `#{period_field}`. Allowed values are #{VALID_PERIOD_FIELDS[issuable_type]}!"
+          unless VALID_PERIOD_FIELDS[issuable_type].include?(period_field.to_sym)
+            raise InvalidPeriodFieldError, "Invalid value for `period_field`: `#{period_field}`. Allowed values are #{VALID_PERIOD_FIELDS[issuable_type].join(', ')}!"
           end
 
           unless period_limit > 0

ee/spec/lib/gitlab/insights/reducers/count_per_period_reducer_spec.rb

@@ -71,7 +71,7 @@ RSpec.describe Gitlab::Insights::Reducers::CountPerPeriodReducer do
     end
 
     it 'raises an error for an unknown :period_field option' do
-      expect { reduce(issuable_relation, period: 'month', period_limit: 5, period_field: :foo) }.to raise_error(described_class::InvalidPeriodFieldError, "Invalid value for `period_field`: `foo`. Allowed values are #{described_class::VALID_PERIOD_FIELDS[:issue]}!")
+      expect { reduce(issuable_relation, period: 'month', period_limit: 5, period_field: :foo) }.to raise_error(described_class::InvalidPeriodFieldError, "Invalid value for `period_field`: `foo`. Allowed values are #{described_class::VALID_PERIOD_FIELDS[:issue].join(', ')}!")
     end
 
     it 'raises an error for an unknown :period_limit option' do
@@ -106,6 +106,10 @@ RSpec.describe Gitlab::Insights::Reducers::CountPerPeriodReducer do
     it 'returns issuables with only the needed fields' do
       expect(reduce(issuable_relation, period: query[:group_by], period_field: :closed_at)).to eq(expected)
     end
+
+    it 'works when string `period_field` is passed' do
+      expect(reduce(issuable_relation, period: query[:group_by], period_field: 'closed_at')).to eq(expected)
+    end
   end
 
   context 'with opened merge requests' do
@@ -122,7 +126,7 @@ RSpec.describe Gitlab::Insights::Reducers::CountPerPeriodReducer do
     end
 
     it 'raises an error for an unknown :period_field option' do
-      expect { reduce(issuable_relation, period: 'month', period_limit: 5, period_field: :foo) }.to raise_error(described_class::InvalidPeriodFieldError, "Invalid value for `period_field`: `foo`. Allowed values are #{described_class::VALID_PERIOD_FIELDS[:merge_request]}!")
+      expect { reduce(issuable_relation, period: 'month', period_limit: 5, period_field: :foo) }.to raise_error(described_class::InvalidPeriodFieldError, "Invalid value for `period_field`: `foo`. Allowed values are #{described_class::VALID_PERIOD_FIELDS[:merge_request].join(', ')}!")
     end
 
     it 'returns issuables with only the needed fields' do