type TimingKind
What a record is: an instant or a closed interval.
The sink needs it and cannot infer it. A zero-length measure is a legitimate
thing to emit, and the two kinds are retained and cleared through different
calls — performance.clearMarks does not touch a measure — so a sink that
guessed from startMs === endMs would leave half of what it emitted behind.
Type
'mark' | 'measure'