Allow stats files to be observed for finishing/unlinking.
Allow stats files to be observed for finishing/unlinking.

This commit adds the `.tss` files generated for TSM statistics to
the `FileObserver` so that package users can be notified when new
stats files are created and removed.

li-ang/influxdb,nooproblem/influxdb,influxdb/influxdb,influxdb/influxdb,nooproblem/influxdb,influxdb/influxdb,influxdb/influxdb,nooproblem/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,li-ang/influxdb,influxdata/influxdb,influxdb/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,influxdata/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb