Add MonitorThreadLeak to TetheringMetricsTest

TetheringMetrics creates a thread to process all metrics work on
a handler. This thread should remain active alongside the
Tethering service but should be closed in test class. Therefore,
add MonitorThreadLeak to ensure the thread is closed correctly.

Test: atest TetheringMetricsTest
Change-Id: Ic57beb3324945ddbb45fec9697056889044e680c
2 files changed