Emit caching metrics for apollo-datasource-rest

[landau]

Package: apollo-datasource-rest

It would be useful to emit metrics around caching. More specifically, some boolean value that indicates when a request results from the cache vs HTTP. In this way, I would be able to emit metrics for my application and adjust TTLs based on those statistics https://github.com/apollographql/apollo-server/blob/master/packages/apollo-datasource-rest/src/HTTPCache.ts#L116-L146

Thank you

[glasser]

We're going to solve #41 by making each fetch call return information about whether or not the call was cached. You can override the protected fetch method in your subclass to look at this information and feed it into your metrics system if you'd like. So I'm considering this (old, sorry!) issue to be a duplicate of #41.