1. OtlpLayer is now always installed (even when export_traces and
   export_logs are both false) so that on_new_span populates
   SpanFields for exemplar capture. on_close/on_event still check
   their export flags and return early.
2. Monoio pre-subscriber config warnings use eprintln! instead of
   tracing::warn! (subscriber isn't installed yet, so tracing
   warnings were silently lost).
3. Zero channel_capacity in monoio exporter now warns explicitly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>