[Perf] Perf framework should allow last in-flight operation to complete

[mikeharder]

Ideally, the Java perf framework should allow the last in-flight operation to complete. For example, say duration = 1 but each operation takes 5 seconds to complete. Ideally, the perf framework should run for 5 seconds and report throughput of 0.2 ops/sec. However, the perf framework currently terminates immediately when the duration expires, due to the use of Flux.take(duration):

azure-sdk-for-java/common/perf-test-core/src/main/java/com/azure/perf/test/core/PerfStressProgram.java

Lines 245 to 253 in a043bdd

	return Flux.just(1)
	.repeat()
	.flatMap(i -> test.runAsync().then(Mono.just(1)), 1)
	.doOnNext(v -> {
	completedOperations[index]++;
	lastCompletionNanoTimes[index] = System.nanoTime() - startNanoTime;
	})
	.take(Duration.ofNanos(endNanoTime - startNanoTime))
	.then();

Here's what we want this code to do:

1. Call test.runAsync() and block until complete
2. Increment completed operations
3. Update lastCompletionNanoTimes
4. If duration has not expired, goto 1

[mikeharder]

Fixed by: #21030