chore: add metrics for mailjet

[utsabc]

What are the changes introduced in this PR?

Currently we have no observability for how much data is being batched for mailjet hence we are adding metrics for the same.

[devops-github-rudderstack]

Test report for this run is available at: https://test-integrations-dev.s3.amazonaws.com/integrations-test-reports/rudder-transformer/3917/test-report.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.47%. Comparing base (b3f7140) to head (4b61508).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3917   +/-   ##
========================================
  Coverage    90.47%   90.47%           
========================================
  Files          615      615           
  Lines        32329    32331    +2     
  Branches      7682     7680    -2     
========================================
+ Hits         29250    29252    +2     
- Misses        2823     2855   +32     
+ Partials       256      224   -32     

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[sonarqubecloud]

Quality Gate failed

Failed conditions
2 New Code Smells (required ≤ 1)

See analysis details on SonarQube Cloud

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