Enhance the stats reported from the Pure Volume Drivers

Enhance the stats reported from the Pure Volume Drivers

This change provides some improvements for older scheduler hints and
stats along with a few new stats that can be used for filtering and
goodness functions. These new stats include:

Latency:
    usec_per_read_op  - Average arrival-to-completion time, measured in
                        microseconds, for a host read operation.
    usec_per_write_op - Average arrival-to-completion time, measured in
                        microseconds, for a host write operation.
    queue_depth       - Average number of queued I/O requests.

IOPS:
    reads_per_sec  - Number of read requests processed per second.
    writes_per_sec - Number of write requests processed per second.
                     Bandwidth
    input_per_sec  - Number of bytes read per second.
    output_per_sec - Number of bytes written per second.

Metadata:
    total_hosts     - Total number of Purity hosts created.
    total_snapshots - Total number of snapshots (includes pending
                      deletion).
    total_pgroups   - Total number of Protection Groups (includes
                      pending deletion).

Note: All of these stats are for the entire array and are not specific
to the Cinder volumes, snapshots, hosts, latency, etc.

As part of the new and improved stats the drivers will respect the
cinder.conf backend “reserved_percentage” option. Previously it was set
to 0.

Last but not least there is a new config option called
“pure_automatic_max_oversubscription_ratio”. This defaults to True, and
when enabled gives the older behavior of automatically calculating the
thin provisioning ratio which was allowed for the backend. If disabled
the driver will now respect the “max_over_subscription_ratio” config
option.

DocImpact
Change-Id: I733f4f90addc39f4c1b720ce63100b15b0523b3e
Implements: blueprint pure-enhanced-stats