SDK: Track all feature flag calls

**Is your feature request related to a problem? Please describe.**

I have been doing some [work on the Go SDK](https://github.com/Flagsmith/flagsmith-go-client/pull/81) and found it weird that the analytics processor only tracked (successful) requests to feature names that are present in the Environment/API. I think it would be interesting to also track requests that are handled by the default flag handler, and those that fail/don't exist.

**Describe the solution you'd like**

Be able to see statistics for all feature flag requests.

**Describe alternatives you've considered**

I might perhaps be uninteresting to track failed requests, but then I do believe it would be of value to track requests that go to the default handler at least.