- clock is needed because timestamps need to be provided now
- explicit types are good anyway
- have at least one helper to emit the signal with optional
tracer/logger/metrics_emitter
- easier logger with `log` and `logf`
follow more closely the official OTEL recommendations, and also try
to reduce global state.
- use a class type for `Exporter.t` (instead of 1st class module `backend`)
- have tracer, logger, metrics_emitter as explicit objects
- keep a `Main_exporter` to make migration easier, but discouraged
- add stdout_exporter and debug_exporter to opentelemetry.client