Feature request: allow to use minitrace without global collector

[andylokandy]

Yeah, that makes perfect sense when compiling and running on a binary with a long-living session.
In our use case, we are building WASM for CloudFlare Worker, where every incoming request has a short-living session. So tracing should start when a request is initiated, and flush the traces when request handling is done.

We can't really use threads, so we want to collect spans in memory, and flush them at the end of the request.

Originally posted by @dotansimha in #179 (comment)