Address comments from code review

Author: pirgeo

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/export/dynatrace/DynatraceProperties.java

@@ -20,13 +20,15 @@
 import org.springframework.boot.actuate.autoconfigure.metrics.export.properties.StepRegistryProperties;
 import org.springframework.boot.context.properties.ConfigurationProperties;
 
+import java.util.Collections;
 import java.util.Map;
 
 /**
  * {@link ConfigurationProperties @ConfigurationProperties} for configuring Dynatrace
  * metrics export.
  *
  * @author Andy Wilkinson
+ * @author Georg Pirklbauer
  * @since 2.1.0
  */
 @ConfigurationProperties(prefix = "management.metrics.export.dynatrace")
@@ -156,7 +158,7 @@ public void setEnrichWithOneAgentMetadata(Boolean enrichWithOneAgentMetadata) {
 	}
 
 	public Map<String, String> getDefaultDimensions() {
-		return defaultDimensions;
+		return defaultDimensions != null ? Collections.unmodifiableMap(defaultDimensions) : null;
 	}
 
 	public void setDefaultDimensions(Map<String, String> defaultDimensions) {

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/metrics/export/dynatrace/DynatracePropertiesConfigAdapterTests.java

@@ -21,13 +21,15 @@
 import org.junit.jupiter.api.Test;
 
 import java.util.HashMap;
+import java.util.Map;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
  * Tests for {@link DynatracePropertiesConfigAdapter}.
  *
  * @author Andy Wilkinson
+ * @author Georg Pirklbauer
  */
 class DynatracePropertiesConfigAdapterTests {