Tracing convention: start_span + set_span_tag/set_span_data helpers

Directive (Burak Yigit Kaya, 2026-07-30): for instrumentation, use with start_span(name="event_manager.save", op="save") as span: and tag/data helpers set_span_tag(span, "platform", platform) / set_span_data(span, "rows_count", len(rows)). op is the category (db, http.client, save, etc.); name is the specific identifier. Tags are indexed/searchable (low cardinality enums); data is high-cardinality context. Pair with the structured-logging convention in the same function.