Avoid ConcurrentModificationException when removing metrics from CassandraMetricsRegistry

Multiple threads can modify the lists in CassandraMetricsRegistry ALIASES map values. We have seen this CME when removing metrics, but could possibly affect other paths using those lists

Adding a small delay (10ms) when map:ing to getMetricName in CassandraMetricsRegistry.remove(name) makes this reproduce when running TransientRangeMovementTest.testLeave

Caused by CASSANDRA-14572