Update DataDog dep. to the most recent release

The original implementation was made when 0.16.0 was the latest version.
Since there there have been a few improvements and bug fixes made to the
library that we should be using.

Reading through the release log there shouldn't be any feature-breaking
changes so we should be good to update it!

tox.ini

@@ -36,7 +36,7 @@ deps=
   selenium==3.0.2
   pymongo==3.4.0
   toml<2.0.0
-  datadog==0.16.0
+  datadog==0.22.0
 passenv =
   USER JAVA_HOME POSTGRES_USER DATAPROC_TEST_PROJECT_ID GCS_TEST_PROJECT_ID GCS_TEST_BUCKET GOOGLE_APPLICATION_CREDENTIALS TRAVIS_BUILD_ID TRAVIS TRAVIS_BRANCH TRAVIS_JOB_NUMBER TRAVIS_PULL_REQUEST TRAVIS_JOB_ID TRAVIS_REPO_SLUG TRAVIS_COMMIT CI
 setenv =