OpenTelemetry Protocol (OTLP) sinks for the obs SDK.
Phase-3 surface (impl-plan task 3.8): the mapping from
ObsEnvelope to OTLP types is implemented + tested. The transport
is pluggable through the [OtlpExporter] trait so users can wire a
tonic::Channel, an HTTP/JSON exporter, or any custom sink. The
built-in [StdoutDebugExporter] writes JSON to stdout — useful for
apps/server end-to-end demos and for the sink's unit tests.
Spec 20 §§ 2.3, 2.4, 2.5 (OTLP mapping) + § 4 (transport).