startSpan abstract method
Creates a child span within a trace.
The parentId should be a trace ID from startTrace.
The spanName should describe the sub-operation
(e.g., "query.execution").
Returns a Span object containing span ID.
Thread Safety: The returned Span object is not thread-safe. Do NOT share across isolates.
Implementation
Span startSpan({
required String parentId,
required String spanName,
Map<String, String> initialAttributes = const {},
});