StatsEngine#metrics usage in tests

[jasonrclark]

Hey there! We noticed that the tests for your gem are using the StatsEngine#metrics method, which with version 3.11.0 of newrelic_rpm is going away.

Luckily, we've provided a replacement which is what we use internally for validating metric recording called assert_metrics_recorded. You can see more documentation for it at https://docs.newrelic.com/docs/agents/ruby-agent/frameworks/third-party-instrumentation#testing-your-extension.

Let us know if you have any problems with updating to use the new testing methods, and thanks for your work extending the Ruby agent! :sparkles:

cc/ @tkrajcar

[josephpage]

cc @flyerhzm