Speed up C++ logging for many individual log calls #4287
Description
The recent performance improvement
got the C++ sdk a lot faster. But compared to Rust we're still behind for individual log calls (like time series scalars!).
An obvious candidate to improve is not building & sending the schema every time: Right now on every log call we convert the schema to C FFI and then create a Rust/arrow2 representation from it. Add a simple lazy schema registry/handle system for this!
Should do a little bit more profiling though to get an idea where the perf goes. E.g. there's likely many many other needless allocs on the way.