Add code coverage using codecov.io

[Brandon-Kimberly]

This is my cut at integrating code coverage with the OpenTelemetry C++ API/SDK repository using codecov.io.

Code coverage tools incentivize developers to write tests and increase coverage. Codecov.io adds additional functionality such as easy-to-use coverage visualizations, PR coverage comment bots, and commit status badges.

I added a new job to the GitHub Actions CI file that runs on every commit and PR against the master branch titled codecov_upload. This job executes a new branch I added to the do_ci.sh file, titled code.coverage. This branch builds the project using CMake, runs CMake tests, generates a coverage report using lcov, then uploads that report to codecov.io.

I used the default YAML config file (.github/.codecov.yaml) with two adjustments.

1. Increased the “green” or ideal coverage lower-bound from 70% to 80%.
2. Included relative file path fixes required by codecov.io to process the report.

Current coverage data for the repository can be found here. Please check it out and let me know if all of the correct files are included.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c86794e). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #121   +/-   ##
=========================================
  Coverage          ?   93.60%           
=========================================
  Files             ?       66           
  Lines             ?     1564           
  Branches          ?        0           
=========================================
  Hits              ?     1464           
  Misses            ?      100           
  Partials          ?        0           

[reyang]

LGTM.