[fix][broker] fix precision error in UsageUnit

[Jason918]

Motivation

UsageUnit.Kbps.convertUnit is always 0. We should use double.

Modifications

Change int to double.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• LinuxInfoUtilsTest

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)
• The rest endpoints: (no)
• The admin cli options: (no)
• Anything that affects deployment: (no)

Documentation

Check the box below and label this PR (if you have committer privilege).

Need to update docs?

• no-need-doc
  bug fix.

[Jason918]

@mattisonchao @michaeljmarshall @gaozhangmin PTAL

[Jason918]

/pulsarbot run-failure-checks