Ensure Datadog::Statsd is loaded during test

[marcotc]

This PR fixes this flaky metrics_spec.rb test: https://app.circleci.com/jobs/github/DataDog/dd-trace-rb/52268

The failure happens because a few tests reference Datadog::Statsd in their before steps without ensuring that it has been loaded:

dd-trace-rb/spec/ddtrace/metrics_spec.rb

Line 166 in 97e8e0d

let(:custom_statsd) { instance_double(Datadog::Statsd) }

More often then not, Datadog::Statsd gets dynamically loaded by metrics.rb

dd-trace-rb/lib/ddtrace/metrics.rb

Line 43 in 97e8e0d

require 'datadog/statsd' unless defined?(::Datadog::Statsd)

and tests run fine, but we shouldn't depend on the side-effects of other tests for a correct test run.

Because I explicitly require 'datadog/statsd' in the test file, I've added a new test to ensure that our code that dynamically loads Datadog::Statsd still works.