usage_test: fix race condition
usage_test: fix race condition

the reporter runs in a ticker in a goroutine and concurrently access
the fake aggmetrics along with the various assertions

raintank/metrictank,replay/metrictank,replay/metrictank,raintank/metrictank,OOM-Killer/metrictank,raintank/metrictank,raintank/raintank-metric,raintank/metrictank,replay/metrictank,replay/metrictank,OOM-Killer/metrictank,raintank/raintank-metric,raintank/raintank-metric,OOM-Killer/metrictank