[feat][monitor] Add offloader stats grafana dashboard

[dao-jun]

Motivation

Add grafana dashboard of offloader stats which introduced by #13833

Modifications

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

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This change added tests and can be verified as follows:

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• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

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

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository:

[lhotari]

Closing and reopening to get Pulsar CI unblocking changes included.

[lhotari]

LGTM

[lhotari]

Thanks for contributing, @dao-jun

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.19%. Comparing base (bbc6224) to head (e48e2f2).
⚠️ Report is 1755 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #23479      +/-   ##
============================================
+ Coverage     73.57%   74.19%   +0.62%     
- Complexity    32624    34331    +1707     
============================================
  Files          1877     1943      +66     
  Lines        139502   146973    +7471     
  Branches      15299    16191     +892     
============================================
+ Hits         102638   109053    +6415     
- Misses        28908    29494     +586     
- Partials       7956     8426     +470     

Flag	Coverage Δ
inttests	27.62% <ø> (+3.04%)	⬆️
systests	24.43% <ø> (+0.10%)	⬆️
unittests	73.54% <ø> (+0.70%)	⬆️

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