event property
event (if non-empty) should be the stable name of some notable moment in the lifetime of a Span. For instance, a Span representing a browser page load might add an Event for each of the Performance.timing moments here: https://developer.mozilla.org/en-US/docs/Web/API/PerformanceTiming
While it is not a formal requirement, Event strings will be most useful if they are not unique; rather, tracing systems should be able to use them to understand how two similar Spans relate from an internal timing perspective.
Implementation
final String event;