Count total data points in a measurement

__Proposal:__
Count all the data points in a measurement and return the value.

__Current behavior:__
This is not possible.

__Desired behavior:__
Something like:
`SHOW DATA_POINTS COUNT FROM cpu;`

__Use case:__
I'm currently profiling usage on a large influxdb instance and counting the rough size of each measurement would help determine where most of our data is coming from.

The issue names no files, tests, or entry points, so begin by locating the measurement-query handling and existing SHOW command tests in InfluxDB. Define the query behavior and verify that the returned count covers all data points in the selected measurement.

Written by the indexing model from the issue text.