Add deletemarker_total metric

[krisis]

Description

Add prometheus metrics to monitor total number of delete markers in a bucket and cluster-wide.

# HELP minio_cluster_usage_deletemarker_total Total number of delete markers in a cluster
# TYPE minio_cluster_usage_deletemarker_total gauge
minio_cluster_usage_deletemarker_total{server="minio1:9000"} 1

# HELP minio_bucket_usage_deletemarker_total Total number of delete markers
# TYPE minio_bucket_usage_deletemarker_total gauge
minio_bucket_usage_deletemarker_total{bucket="bucket",server="minio1:9000"} 1

How to test this PR?

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

[vadmeste]

LGTM