[FEATURE] Standardize, setup, report, and increase test coverage

[Xtansia]

Setup code coverage, once reporting from CI/CD analyze the test gap and fill it.

See the meta issue opensearch-project/opensearch-clients#30 for details.

[DiyazY]

Hi. I can look at this. Is it ok if we close this issue with several PRs?
For example: 1st PR is setting up the test coverage, 2nd or Nth PRs are about increasing the test coverage. I think it will make the review process much easier.

Do we have any preferred test coverage tool? I assume it is codecov.

Tag: CCI

[dblock]

No preference on coverage tool.

[DiyazY]

@dblock Thank you! I will take this issue.

[tejeshreddy]

@dblock could you please provide more information on this issue? The current code coverage is 96% for this repo, does this issue expect to fill the gaps where the tests aren't covered?

[dblock]

Looks like this was done, closing.