minor: tidy up opentelemetry code

[abr-egn]

This pulls in a few tidy-ups from my tracing spans branch (that I'm calling Not Worth The Effort):

• .with_current_context() is only actually needed if you're putting the future on a different task, which in turn means otel_stub isn't needed at all.
• created a thin wrapper around with_context so OpSpan.context can stay private to the otel module.