[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.

[Yury-Fridlyand]

I did an investigation in this area recently.
My findings so far:

• Tools for testing code coverage in .net:
  • https://code-maze.com/dotnet-code-coverage/
  • https://docs.microsoft.com/en-us/dotnet/core/testing/unit-testing-code-coverage
  • SonarQube
• Current code coverage is about 20-30%

[Xtansia]

From my past experience with SonarQube/SonarCloud, it does not collect code coverage itself, you'd use something like https://github.com/coverlet-coverage/coverlet or https://www.jetbrains.com/dotcover/ to do the actual collection, and then submit the coverage stats to SonarQube/SonarCloud for reporting and use in quality gates.

It appears the clients and other OpenSearch projects already reporting coverage statistics are using codecov.io as the reporting service: https://github.com/search?l=YAML&p=1&q=org%3Aopensearch-project+codecov&type=Code.