Add code coverage tool to build

[matrei]

Issue description

For example: https://docs.gradle.org/current/userguide/jacoco_plugin.html

[jdaugherty]

We should investigate how groovy reports on code coverage as well. It posts to Prs to confirm if code coverage goes up or down.

[jamesfredley]

Also see Codecov Comments Bot used by Apache Groovy.

apache/groovy#2385 (comment)

[jdaugherty]

To get started, we could copy the coverage options from the server-timing project. We may need a separate build though as it has a bit of overhead.