refactor(SdkCommon): Keep static userAgent value

Author: lpatino10

common/src/main/java/com/ibm/watson/common/SdkCommon.java

@@ -12,6 +12,7 @@
 
 public class SdkCommon {
   private static final Logger LOG = Logger.getLogger(SdkCommon.class.getName());
+  private static String userAgent;
 
   private static String loadSdkVersion() {
     ClassLoader classLoader = SdkCommon.class.getClassLoader();
@@ -28,7 +29,10 @@ private static String loadSdkVersion() {
   }
 
   private static String getUserAgent() {
-    return "watson-apis-java-sdk/" + loadSdkVersion() + "; " + RequestUtils.getUserAgent();
+    if (userAgent == null) {
+      userAgent = "watson-apis-java-sdk/" + loadSdkVersion() + "; " + RequestUtils.getUserAgent();
+    }
+    return userAgent;
   }
 
   public static Map<String, String> getDefaultHeaders(String serviceName, String serviceVersion, String operationId) {

common/src/main/java/com/ibm/watson/common/WatsonHttpHeaders.java

@@ -0,0 +1,6 @@
+package com.ibm.watson.common;
+
+public interface HttpHeaders {
+  /** Header containing analytics info. */
+  String X_IBMCLOUD_SDK_ANALYTICS = "X-IBMCloud-SDK-Analytics";
+}