ignore disk metrics for single drive mode

[harshavardhana]

Description

ignore disk metrics for single drive mode

Motivation and Context

fixes #14211

How to test this PR?

~ curl https://localhost:9000/minio/v2/metrics/node

Can crash the single drive server setup with Prometheus.

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 yes introduced in prometheus: Publish XL storage API latency #14117
• Documentation updated
• Unit tests added/updated

[minio-trusted]

Mint Automation

Test	Result
mint-large-bucket.sh	✔️
mint-fs.sh	✔️
mint-gateway-s3.sh	✔️
mint-erasure.sh	✔️
mint-dist-erasure.sh	✔️
mint-gateway-nas.sh	✔️
mint-compress-encrypt-dist-erasure.sh	✔️
mint-pools.sh	✔️
Deleting image on docker hub
Deleting image locally