[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 https://github.com/opensearch-project/opensearch-clients/issues/30 for details.

[Xtansia]

codecov.io is already in use by other clients and opensearch projects for reporting, so that is essentially settled.

To do the actual coverage collection, it appears the main options are directly with the compiler https://doc.rust-lang.org/rustc/instrument-coverage.html or with https://github.com/xd009642/tarpaulin.

It appears to me that the compiler's support is the new best practice, and using something like grcov to get lcov output to pass to codecov

[harshavamsi]

@Xtansia this can be closed.

[Xtansia]

@harshavamsi This issue is not complete, only the report part has actually been completed