Explicitly require `active_support/core_ext/kernel/singleton_class` when needed
Explicitly require `active_support/core_ext/kernel/singleton_class` when needed

MetricsGlobalTest is using a `class_eval` which requires the `singleton_class`.

buttercloud/active_merchant,vanboom/active_merchant,curiousepic/active_merchant,varyonic/active_merchant,vampirechicken/active_merchant,Shopify/active_merchant,pinpayments/active_merchant,Simplero/active_merchant,activemerchant/active_merchant,atxwebs/active_merchant