Introduce cache lookup instrumentation hook

[danielhartnell]

Relates to: #124

This attempts to determine if there was a cache hit and then records that along with cache key, operation name and path attributes.

By default, this calls cache_lookup_event() which has no action.

Implementing the gem in the parent application would enable monitoring (or whatever) of these cache hits. In our case, we probably just want to increment a cache hit or miss metric tagged with the path and operation name.

module GraphQL
  module FragmentCache
    class Fragment
      def self.cache_lookup_event(**args)
        # Monitoring such as incrementing cache hit counter
      end
    end
  end
end

[DmitryTsepelev]

LGTM! Could you please make Rubocop pass and maybe add something to docs before I merge it in? 🙂

[danielhartnell]

Happy New Year @DmitryTsepelev! The Rubocop violation was fixed and I included some documentation in the readme. It felt reasonable to make this a configurable feature so I disabled it by default and added the ability to enable it, like caching itself, via GraphQL::FragmentCache.monitoring_enabled.

Let me know if you'd like me to change anything.

[DmitryTsepelev]

LGTM! Thank you so much and happy new year!