Add Badge for Code Coverage

[kxyr]

Is your feature request related to a problem?
Most of the existing OpenTelemetry repositories display a badge for code coverage. In an effort to have the Rust SDK repository to be up to date and consistent with the other established SDKs and repositories, there should be a code coverage badge in the main README.md file.

Describe the solution you'd like.
As a developer contributing to OpenTelemetry, I recommend adding a code coverage percentage badge at the top of the README document of the Rust repo. Code coverage badges are a common feature of many modern open source projects, which improves readability and convenience to developers. By adding a code coverage badge to the README.md, with a quick scan, any observer will be able to know the status of the repository.

cc: @alolita