be more clear about what the mutex protects
be more clear about what the mutex protects

it's a best practice to put the mutex above the fields it protects
see https://hackernoon.com/dancing-with-go-s-mutexes-92407ae927bf
or https://golang.org/src/log/log.go#L50

raintank/metrictank,raintank/metrictank,replay/metrictank,raintank/raintank-metric,replay/metrictank,raintank/metrictank,replay/metrictank,raintank/metrictank,replay/metrictank,raintank/raintank-metric,raintank/raintank-metric