Add namespace for headers. (#954)

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/conf/Config.java

@@ -31,6 +31,11 @@ import org.apache.skywalking.apm.agent.core.logging.core.WriterFactory;
 public class Config {
 
     public static class Agent {
+        /**
+         * Namespace isolates headers in cross process propagation. The HEADER name will be `HeaderName:Namespace`.
+         */
+        public static String NAMESPACE = "";
+
         /**
          * Application code is showed in sky-walking-ui. Suggestion: set an unique name for each application, one
          * application's nodes share the same code.

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/CarrierItem.java

@@ -16,10 +16,12 @@
  *
  */
 
-
 package org.apache.skywalking.apm.agent.core.context;
 
 import java.util.Iterator;
+import org.apache.skywalking.apm.util.StringUtil;
+
+import static org.apache.skywalking.apm.agent.core.conf.Config.Agent.NAMESPACE;
 
 /**
  * @author wusheng
@@ -30,13 +32,15 @@ public class CarrierItem implements Iterator<CarrierItem> {
     private CarrierItem next;
 
     public CarrierItem(String headKey, String headValue) {
-        this.headKey = headKey;
-        this.headValue = headValue;
-        next = null;
+        this(headKey, headValue, null);
     }
 
     public CarrierItem(String headKey, String headValue, CarrierItem next) {
-        this.headKey = headKey;
+        if (StringUtil.isEmpty(NAMESPACE)) {
+            this.headKey = headKey;
+        } else {
+            this.headKey = NAMESPACE + ":" + headValue;
+        }
         this.headValue = headValue;
         this.next = next;
     }