admin log:add log level filter

[dormanze]

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Description

When I run the admin log command to view the server logs, I find that some empty logs are often printed. For example:

Motivation and Context

We want to view the event logs of the minio, but the printed result is empty.
ps:The event-level logs of nodes that fall out of the cluster and then rejoin the cluster are helpful.

How to test this PR?

mc admin log myminio --level all

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