A single unit of work within a trace (db_query | http_call | custom).
Users never build these — instrumentation creates them. Serialized to the exact
ingest wire shape.
Summary
Functions
Build a span. type is "db_query" | "http_call" | "custom". started_at is an
ISO8601 string. meta is an optional map (e.g. %{"rowCount" => n}).
Serialize to the ingest wire shape (string keys, camelCase).