Differentiate metric latency measured in millis and nanos

Today most of the client latency metrics are measured in millis, and some in nanos. For those measured in nanos we usually differentiate them by having a `-ns` suffix in the metric names, e.g. `io-wait-time-ns-avg` and `io-time-ns-avg`. But there are a few that we obviously forgot to follow this pattern, e.g. `io-wait-time-total`: it is inconsistent where `avg` has `-ns` suffix and `total` has not. I did a quick search and found just three of them:

• bufferpool-wait-time-total -> bufferpool-wait-time-ns-total
• io-wait-time-total -> io-wait-time-ns-total
• iotime-total -> io-time-ns-total (note that there are two inconsistencies on naming, the average metric is `io-time-ns-avg` whereas total is `iotime-total`, I suggest we use `io-time` instead of `iotime` for both).

We should change their name accordingly with the `-ns` suffix as well.