capture I/O in waiting and total tokens in diskMetrics

[harshavardhana]

Community Contribution License

All community contributions in this pull request are licensed to the project maintainers
under the terms of the Apache 2 license.
By creating this pull request I represent that I have the right to license the
contributions to the project maintainers under the Apache 2 license.

Description

capture I/O in waiting and total tokens in diskMetrics

Motivation and Context

This is needed for the subsequent changes
in ServerUpdate(), ServerRestart() etc.

How to test this PR?

This change mainly captures additional
metrics, for blocked I/O to be responded
back via DiskInfo() call for top layer
callers to perform certain maintenance
activity.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here

[harshavardhana]

I realize that there are no changes in the code, I have replied to your comments @klauspost please let me know if you want me to rework anything.