Summary:
This adds code coverage with both html output
and tracking/visualization on codecov.io.

Test Plan:
Tested on Jenkins by using a remote branch,

  Sample output:
  https://codecov.io/gh/pixie-labs/pixielabs/tree/9206c22a8cfa8b675a675ae82a1a109e36e9f91e
  You will need to login with github (and grant access).

Reviewers: #engineering, michelle, oazizi

Reviewed By: #engineering, michelle, oazizi

Subscribers: kgandhi

Differential Revision: https://phab.corp.pixielabs.ai/D309

GitOrigin-RevId: 2f1c0d83891c3e8d1f653a6712110ca3a4104283