Enable aggregate level IOPS in RHEL for Zapi

[rahulguptajss]

1. Create a file custom.yaml if not exist here. In harvest directory, it is conf/zapiperf
2. Add below content in this file

objects:
  Volume:                 custom_volume.yaml

3. Create a new file custom_volume.yaml here. In harvest directory, it is conf/zapiperf/cdot/9.8.0

4. update custom_volume.yaml as below

plugins:
  - Aggregator:
      - aggr<>aggr
    Max:
      - aggr<>aggr_max

5: Restart harvest
6: Wait for few minutes and you should see below metrics in prometheus.

All metrics exposed by template will also be available with prefixes aggr and aggr_max

Note:
There are some fixes in how average latencies are calculated as part of #1666 . It is recommended to use a build with this fix for average latency metrics (for example at node,aggr level).

More details about object templates here
For Rest Collectors, refer steps here
For NABox, follow similiar steps as mentioned in #1284. But template changes as mentioned in here #1740