Remove range slice data from read latency metric calculation

This was causing read latency graphs to not match up with Opscenter.
Range slice shoould have separate metric data instead.

Typical read metric data from an idle cluster pre-patch:
[3.9878362E-4,2,0.0,3.9878362E-4,1.5902837E-7]

Typical read metric data from an idle cluster post-patch:
[6.023493E-4,1,6.023493E-4,6.023493E-4,3.6282466E-7]

The count of 2 was causing the average and minimum values to be off.

https://docs.newrelic.com/docs/plugin-developer-resources/metric-data-for-the-plugin-api#timeslice
"Count of the number of events this value represents over the time
period; the average is calculated by dividing total by count"
Merge pull request #29 from jbarbuto/remove_range_slice_from_read_latency

Remove range slice data from read latency metric calculation