Fix another spec

Author: DouweM

spec/lib/gitlab/metrics/method_call_spec.rb

@@ -23,7 +23,7 @@
 
       expect(metric.values[:duration]).to be_a_kind_of(Numeric)
       expect(metric.values[:cpu_duration]).to be_a_kind_of(Numeric)
-      expect(metric.values[:call_count]).to an_instance_of(Integer)
+      expect(metric.values[:call_count]).to be_an(Integer)
 
       expect(metric.tags).to eq({ method: 'Foo#bar' })
     end