Enhance thread context uniqueness assertion

This commit enhances the assertion message when de-duplicating values on
a thread context header so that if this assertion trips we can see the
values and their correpsonding unique values to understand why this
assertion might trip.

Author: jasontedor

core/src/main/java/org/elasticsearch/common/util/concurrent/ThreadContext.java

@@ -450,7 +450,8 @@ private ThreadContextStruct putResponse(final String key, final String value, fi
 
             if (existingValues != null) {
                 final Set<String> existingUniqueValues = existingValues.stream().map(uniqueValue).collect(Collectors.toSet());
-                assert existingValues.size() == existingUniqueValues.size();
+                assert existingValues.size() == existingUniqueValues.size() :
+                        "existing values: [" + existingValues + "], existing unique values [" + existingUniqueValues + "]";
                 if (existingUniqueValues.contains(uniqueValue.apply(value))) {
                     return this;
                 }