Add valuesWritten stat

__Proposal:__
Add a `valuesWritten` metric that is a running total of all values written.

__Current behavior:__
There is a `pointsWritten` metric, which will continue to function as is. `pointsWritten` is a running total of points written and each of those points can contain multiple fields (values). While that is a useful metric, we also need total values written, as that is what impacts write throughput most. E.g., the following write is 1 point and 2 values.
```
cpu idle=20.0,user=30.0
```

__Desired behavior:__
The example write above would increment `pointsWritten` by 1 and the new `valuesWritten` metric by 2.

__Implementation notes:__
There was some discussion of having [this area](https://github.com/influxdata/influxdb/blob/master/models/points.go#L428) keep a running total and pass it back from `ParsePoints()`. However, this metric will need to come from a lower level so that replicated writes in enterprise also get counted. [Probably here](https://github.com/influxdata/influxdb/blob/1.8/tsdb/store.go#L1374).