Add code coverage section for the library

[Shivamdhar]

We should be tracking code coverage for the client library to ensure we are not missing test cases for any new features or a module added in the past. This should be shown as a tag in the README section alongside CI and IntegrationTests status.

Follow https://app.codecov.io/gh/opensearch-project/opensearch-py/ for setup and further instructions.
Example repo for reference: https://github.com/codecov/example-python

[anasalkouz]

Anyone tracking this? is there a reason why you are keeping this issue untriaged for longtime?

[dblock]

Yes, this should be a child of opensearch-project/opensearch-clients#30, we still need code coverage. cc: @wbeckler

[bl1nkker]

Hello!

I looked through the rest of the repositories and realized that opensearch-py is one of the repositories where codecov is not integrated. For this reason, it is not possible to visualize test coverage.

I can solve this problem. Before that, I had to work with codecov integration on my personal projects.