Spun out from SERVER-123982. There I added some strange behavior to MetricsService to allow external callers to own the metrics. This was due to server_status_servers_test.cpp calling 'clearForTests()', which would delete the unique_ptrs that I wanted to capture as global variables. The test would then cause segfaults whenever a command ran and tried to increment the global opCounters which pointed to the previous metrics service instantiation (deleted), a use after free.

mike.nugent@mongodb.com suggested this could be improved and simplified.