A possible implementation that exposes some cache metrics.

[stevengssns]

Hello,

at the project I'm working on in my day job we are using this library already for a while in production. It is doing a fine job so far, but we are starting to encounter a few limitations.

So I have made an attempt at tackling the first and easiest one of them. Exposing information about how the cache was used when receiving a fetch result.

I have done a quick test using our project code, and everything seems to be working. But before I try to add the necessary unit tests, I would of course first like to get feedback about this solution.

At least partially fixes #41

[apollo-cla]

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