end(requestId, errorCode) {
		this.totalRequests += 1
		if (!this.requests.has(requestId)) {
			return -2;
		}
		if (!this.loadTest.running) {
			return -1;
		}
		const elapsed = this.getElapsedMs(this.requests.get(requestId));
		this.add(elapsed, errorCode);
		this.requests.delete(requestId);
		return elapsed;
	}
Compute elapsed time and add the measurement.
Accepts an optional error code signaling an error.