ZOOKEEPER-4252: Fix flaky test RequestPathMetricsCollectorTest#testMultiThreadPerf

[kezhuw]

This test is asserting low performance threshold for RequestPathMetricsCollector#registerRequest. It fails at rate 20% according to jira reports. I think it caused by combination of:

1. Unstable test environment.
2. Executors.newCachedThreadPool is not suitable for cpu-bound operations.
3. System.currentTimeMillis is not suitable for time counting due to possible wall clock skewing.

This pr try to fix flakyness through:

1. Increase timeout for unstable test environment.
2. Use Executors.newWorkStealingPool to execute register operation.
3. Use System.nanoTime to count elapsed time.

Additionally, it logs elapsed time for performance measurement.