Add missing import

Author: chlos

testing/agent-for-testing/src/test/java/io/opentelemetry/javaagent/testing/AgentForTestingTest.java

@@ -9,6 +9,7 @@
 
 import io.opentelemetry.api.GlobalOpenTelemetry;
 import io.opentelemetry.javaagent.testing.common.AgentTestingExporterAccess;
+import io.opentelemetry.sdk.metrics.data.MetricData;
 import io.opentelemetry.sdk.trace.data.SpanData;
 import java.util.List;
 import org.junit.jupiter.api.BeforeEach;