Add unit test for open search client bulk requests

[neil-xie]

What changed?
Refactor bulk add method, separate callback func for better testability
Add unit test for open search client bulk requests

Why?
Unit test converage

How did you test it?

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

Attention: Patch coverage is 78.26087% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 64.06%. Comparing base (2ac145f) to head (98386f4).
Report is 1 commits behind head on master.

Additional details and impacted files

Files	Coverage Δ
common/elasticsearch/client/os2/client_bulk.go	92.38% <78.26%> (+88.67%)	⬆️

... and 6 files with indirect coverage changes

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[coveralls]

Pull Request Test Coverage Report for Build 018f547e-b777-41d6-a318-9218648d0b88

Details

• 24 of 30 (80.0%) changed or added relevant lines in 1 file are covered.
• 26 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.08%) to 68.208%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
common/elasticsearch/client/os2/client_bulk.go	24	30	80.0%

Files with Coverage Reduction	New Missed Lines	%
common/task/weighted_round_robin_task_scheduler.go	1	89.05%
service/history/task/transfer_standby_task_executor.go	2	86.21%
common/task/fifo_task_scheduler.go	2	87.63%
service/matching/taskListManager.go	2	81.16%
service/history/execution/mutable_state_util.go	2	78.52%
service/history/queue/timer_gate.go	3	95.83%
service/history/execution/mutable_state_task_refresher.go	14	64.56%

Totals
Change from base Build 018f5467-b337-442d-bcf0-8ad761c213c7:	0.08%
Covered Lines:	100188
Relevant Lines:	146887

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💛 - Coveralls