Fix servicecomb plugin trace break in cse case (#460)

Author: Chenfx-git

CHANGES.md

@@ -16,6 +16,7 @@ Release Notes.
 * Refactor kotlin coroutine plugin with CoroutineContext.
 * Fix OracleURLParser ignoring actual port when :SID is absent.
 * Change gRPC instrumentation point to fix plugin not working for server side.
+* Fix servicecomb plugin trace break.
 
 #### Documentation
 * Update docs of Tracing APIs, reorganize the API docs into six parts.

apm-sniffer/apm-sdk-plugin/servicecomb-plugin/servicecomb-java-chassis-2.x-plugin/src/main/java/org/apache/skywalking/apm/plugin/servicecomb/v2/ProducerOperationHandlerInterceptor.java

@@ -42,7 +42,14 @@ public void beforeMethod(EnhancedInstance objInst, Method method, Object[] allAr
         CarrierItem next = contextCarrier.items();
         while (next.hasNext()) {
             next = next.next();
-            next.setHeadValue(invocation.getContext().get(next.getHeadKey()));
+            String headKey = next.getHeadKey();
+            if (invocation.getContext().containsKey(headKey)) {
+                // cse invoke scenario
+                next.setHeadValue(invocation.getContext().get(headKey));
+            } else {
+                // not cse invoke scenario
+                next.setHeadValue(invocation.getRequestEx().getHeader(headKey));
+            }
         }
         String operationName = invocation.getMicroserviceQualifiedName();
         AbstractSpan span = ContextManager.createEntrySpan(operationName, contextCarrier);