Add average IO size in storage group metric

[P-Cao]

Description

Add a metric for storage group performance query

GitHub Issues

List the GitHub issues impacted by this PR:

GitHub Issue #
dell/csm#586

Checklist:

• Have you run format,vet & lint checks against your submission?
• Have you made sure that the code compiles?
• Did you run the unit & integration tests successfully?
• Have you maintained at least 90% code coverage?
• Have you commented your code, particularly in hard-to-understand areas
• Have you done corresponding changes to the documentation
• Did you run tests in a real Kubernetes cluster?
• Have you maintained at least 90% code coverage?
• Backward compatibility is not broken

How Has This Been Tested?

UT is covered: