Clean up Monitoring V3 sample.

[tswast]

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.

[waprin]

Will circle back to this as soon as get through launch craziness.

[tswast]

Some test classes are currently failing checkstyle checks.

[INFO] --- maven-checkstyle-plugin:2.17:check (default) @ cloud-monitoring-v3-samples ---
[INFO] Starting audit...
/home/travis/build/GoogleCloudPlatform/java-docs-samples/monitoring/v3/src/test/java/CreateCustomMetricTest.java:19: error: 'org.apache.commons.lang3.RandomStringUtils' should be separated from previous import group.
/home/travis/build/GoogleCloudPlatform/java-docs-samples/monitoring/v3/src/test/java/CreateCustomMetricTest.java:53: error: At-clause should have a non-empty description.
/home/travis/build/GoogleCloudPlatform/java-docs-samples/monitoring/v3/src/test/java/CreateCustomMetricTest.java:66: error: At-clause should have a non-empty description.
/home/travis/build/GoogleCloudPlatform/java-docs-samples/monitoring/v3/src/test/java/ListResourcesTest.java:19: error: 'junit.framework.Assert' should be separated from previous import group.
Audit done.

https://travis-ci.org/GoogleCloudPlatform/java-docs-samples/builds/116512174

I had planned to enable this check (applying checkstyles to test classes) in PR #115.