Tracing data model

The data model of spans is an extension of emit's events. Span events include the following well-known properties:

• evt_kind: with a value of "span" to indicate that the event is a span.
• span_name: a name for the operation the span represents. This defaults to the template.
• span_kind: a hint about the way an operation and its parent are related.
• span_id: an identifier for this specific invocation of the operation.
• parent_id: the span_id of the operation that invoked this one.
• trace_id: an identifier shared by all events in a distributed trace. A trace_id is assigned by the first operation.