Capture API request milestones for analytics

We need to be able to capture API request milestones in Heap for user activation information. We only care about the key SDK endpoints and we'd like to be able to capture when a user* makes their first API request, 5k request, 10k request, 50k requests, etc.

Some problems / considerations:

* Our SDK endpoints are very latency sensitive so adding additional calls to Heap will need to be asyncronously
* API requests are made in the context of an environment, not a user - how do we achieve this in Heap.
* Can we determine whether a request came from an SDK in their own project vs just the 'try it out' button on the FE