Opentelemetry.Span_ctxSpan context. This bundles up a trace ID and parent ID.
https://opentelemetry.io/docs/specs/otel/trace/api/#spancontext
val make : trace_id:Trace_id.t -> parent_id:Span_id.t -> unit -> tval dummy : tInvalid span context, to be used as a placeholder
val is_valid : t -> boolval trace_id : t -> Trace_id.tval is_remote : t -> boolval to_w3c_trace_context : t -> bytesval of_w3c_trace_context_exn : bytes -> t