Allow service name mapping to apply without setting the tag.

Previously the mapping only applied when the `service.name` tag was set, not on the default or configured service name.

dd-trace-ot/src/main/java/datadog/opentracing/DDTracer.java

@@ -57,6 +57,8 @@ public class DDTracer implements io.opentracing.Tracer {
 
   /** A set of tags that are added to every span */
   private final Map<String, String> spanTags;
+  /** A configured mapping of service names to update with new values */
+  private final Map<String, String> serviceNameMappings;
 
   /** Span context decorators */
   private final Map<String, List<AbstractDecorator>> spanContextDecorators =
@@ -116,6 +118,7 @@ public class DDTracer implements io.opentracing.Tracer {
     this.writer.start();
     this.sampler = sampler;
     this.spanTags = defaultSpanTags;
+    this.serviceNameMappings = serviceNameMappings;
 
     try {
       Runtime.getRuntime()
@@ -522,6 +525,10 @@ public class DDTracer implements io.opentracing.Tracer {
         serviceName = DDTracer.this.serviceName;
       }
 
+      if (serviceNameMappings.containsKey(serviceName)) {
+        serviceName = serviceNameMappings.get(serviceName);
+      }
+
       final String operationName =
           this.operationName != null ? this.operationName : this.resourceName;
 

dd-trace-ot/src/test/groovy/datadog/opentracing/decorators/SpanDecoratorTest.groovy

@@ -5,12 +5,14 @@ import datadog.opentracing.DDTracer
 import datadog.opentracing.SpanFactory
 import datadog.trace.api.DDSpanTypes
 import datadog.trace.api.DDTags
+import datadog.trace.common.sampling.AllSampler
 import datadog.trace.common.writer.LoggingWriter
 import io.opentracing.tag.StringTag
 import io.opentracing.tag.Tags
 import spock.lang.Specification
 
 import static datadog.opentracing.DDTracer.UNASSIGNED_DEFAULT_SERVICE_NAME
+import static java.util.Collections.emptyMap
 
 class SpanDecoratorTest extends Specification {
   def tracer = new DDTracer(new LoggingWriter())
@@ -74,6 +76,23 @@ class SpanDecoratorTest extends Specification {
     "other-context" | "my-service"                    | "my-service"
   }
 
+  def "default or configured service name can be remapped without setting tag"() {
+    setup:
+    tracer = new DDTracer(serviceName, new LoggingWriter(), new AllSampler(), emptyMap(), mapping, emptyMap())
+
+    when:
+    def span = tracer.buildSpan("some span").start()
+
+    then:
+    span.serviceName == expected
+
+    where:
+    serviceName                     | expected                        | mapping
+    UNASSIGNED_DEFAULT_SERVICE_NAME | UNASSIGNED_DEFAULT_SERVICE_NAME | ["other-service-name": "other-service"]
+    UNASSIGNED_DEFAULT_SERVICE_NAME | "new-service"                   | [(UNASSIGNED_DEFAULT_SERVICE_NAME): "new-service"]
+    "other-service-name"            | "other-service"                 | ["other-service-name": "other-service"]
+  }
+
   def "set operation name"() {
     when:
     Tags.COMPONENT.set(span, component)