Consolidate drive health related metrics into single metric

[anjalshireesh]

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

Instead of having "online" and "healing" as two metrics, replace with a single metric "health" which can have following values:

0 = offline
1 = healthy
2 = healing

Motivation and Context

better metrics

How to test this PR?

curl "http://<endpoint>/minio/metrics/v3/system/drive"

should return a new metric minio_system_drive_health in place of the two erstwhile metrics minio_system_drive_online and minio_system_drive_healing

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

[donatello]

Using an "enum" here doesn't seem very understandable. If this metric is graphed, it seems more complicated to understand - why do we need to do this? At the very least, it seems better to let the numbers indicate health value (higher => more healthy in a sense):
0 -> offline
1 -> healing
2 -> healthy

[shtripat]

lgtm