Add a log entry to a distributed trace

Creates a new trace span or enriches an existing one. Omit traceId to start a new trace; the returned traceId can then be passed as the traceparent header in subsequent calls to correlate them. Trace IDs are always in W3C traceparent format, e.g. 00-0427951c0c0a82d422f30fbc1b18d1e1-a39b316aa5d96365-01.