init global tracer in premain

dd-java-agent/src/main/java/com/datadoghq/agent/AgentRulesManager.java

@@ -24,6 +24,7 @@ public class AgentRulesManager {
   public AgentRulesManager(final TracingAgentConfig config) {
     agentTracerConfig = config;
     instrumentationRulesManager = new InstrumentationRulesManager(config, this);
+    instrumentationRulesManager.initTracer();
   }
 
   /** This method initializes the manager. */

dd-java-agent/src/main/java/com/datadoghq/agent/InstrumentationRulesManager.java

@@ -120,11 +120,9 @@ public class InstrumentationRulesManager {
     log.info("Initializing on classloader {}", classLoader);
 
     injector.inject(classLoader);
-
-    initTracer();
   }
 
-  private void initTracer() {
+  void initTracer() {
     synchronized (SYNC) {
       if (!GlobalTracer.isRegistered()) {
         // Try to obtain a tracer using the TracerResolver
@@ -135,6 +133,8 @@ public class InstrumentationRulesManager {
           } catch (final RuntimeException re) {
             log.warn("Failed to register tracer '" + resolved + "'", re);
           }
+        } else {
+          log.warn("Failed to resolve dd tracer");
         }
       }
     }