Clean up Monitoring V3 sample. #118

Some minor edits needed for javadocs, also using Truth for some of the assertions would really help in the tests. See PR #116.

• checkstyle needs to pass on the test classes. Fixed by Fix checkstyle in monitoring v3 sample. #122.
• use Truth for assertions in tests. Fixed by Fix checkstyle in monitoring v3 sample. #122.
• enable tests by default, so that they run in Travis. (Depends on whitelisting the cloud-samples-tests project.)
• adjust line breaks to follow "the rectangle rule".
• <p> tags in recommended line in javadocs.
• make license headers regular comment, not javadocs.