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 Ruby 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 Ruby 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

[fbogsany]

Code coverage is evaluated as part of CI for several gems (but not all) and results are written to the filesystem, e.g.

Coverage report generated for Unit Tests to /__w/opentelemetry-ruby/opentelemetry-ruby/exporter/otlp/coverage. 286 / 312 LOC (91.67%) covered.

We use SimpleCov and some gems set the minimum coverage to 85%. E.g. https://github.com/open-telemetry/opentelemetry-ruby/blob/b7149fb72cad6c45956a6be80acfb2d062aab3a6/api/test/test_helper.rb#L7-L9 We don't yet submit the results to anything that could track code coverage for us and produce a badge. I'm a little nervous about the use of codecov.io, for example, after https://about.codecov.io/security-update/

[angeliski]

Hey, @fbogsany We can use Codeclimate, WDYT? I can help here if necessary :)

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