ReadonlytraceReadonlyspanOptional ReadonlytraceOptional ReadonlytraceW3C tracestate (serialized), when the tracer reports one; carried across replicas with the ids.
Optional Readonlyistrue for a context that came from another process (a checkpoint resumed after a
hand-over or a restart, see FlowCheckpoint.trace). Adapters map it to their own flag.
Opaque identity of a span (adapters fill it; the core never parses it).