Annotate with VisibleForTesting

Author: odeke-em

google-cloud-spanner/src/main/java/com/google/cloud/spanner/XGoogSpannerRequestId.java

@@ -16,12 +16,17 @@
 
 package com.google.cloud.spanner;
 
+import com.google.common.annotations.VisibleForTesting;
 import java.security.SecureRandom;
 
 public class XGoogSpannerRequestId {
   // 1. Generate the random process Id singleton.
+  @VisibleForTesting
   public static String RAND_PROCESS_ID = XGoogSpannerRequestId.generateRandProcessId();
+
+  @VisibleForTesting
   public static long VERSION = 1; // The version of the specification being implemented.
+
   private long nthClientId;
   private long nthChannelId;
   private long nthRequest;