Add code coverage report

[ghost]

Is your feature request related to a problem? Please describe.
Currently there is no code coverage published for the project, therefore it's harder to spot areas where testing should be improved.

Describe the solution you'd like
Eg CodeCov integration, such as for https://github.com/open-telemetry/opentelemetry-java

Describe alternatives you've considered
None

Additional context
Code coverage report showing strong quality numbers should also help us in popularisation of the project

[mateuszrzeszutek]

Duplicate of #3046; closing this one because the other issue contains more details