beginner: add coverage

Going to leave open a persistent beginner ticket for adding coverage:
https://s3.amazonaws.com/lyft-envoy/coverage/report-master/coverage.html

There are always a few missing things here and there especially regarding random error handling. If you are a beginner this is a great way to learn and do something valuable.

Rough steps:

- Look at the coverage report and find something (often a single line) that looks like it should be covered.
- Find the relevant unit test that should cover it (often in the test tree with the same file name)
- Write the test
- Confirm in PR coverage build that line is covered. (Coverage report is posted as a build artifact).