Avoid extra allocations in RestGetAliasesAction

When no explicit aliases are provided in the call there is no need
to collect the index names or aliases into HashSets if they won't be
used.

Also fixed where the index name was being added for each loop of the
alias list.

Author: JeremyDahlgren

server/src/main/java/org/elasticsearch/rest/action/admin/indices/RestGetAliasesAction.java

@@ -84,19 +84,25 @@ static RestResponse buildRestResponse(
     ) throws Exception {
         final Set<String> indicesToDisplay = new HashSet<>();
         final Set<String> returnedAliasNames = new HashSet<>();
-        for (final Map.Entry<String, List<AliasMetadata>> cursor : responseAliasMap.entrySet()) {
-            for (final AliasMetadata aliasMetadata : cursor.getValue()) {
+        if (aliasesExplicitlyRequested || requestedAliases.length > 0) {
+            for (final Map.Entry<String, List<AliasMetadata>> cursor : responseAliasMap.entrySet()) {
                 if (aliasesExplicitlyRequested) {
                     // only display indices that have aliases
                     indicesToDisplay.add(cursor.getKey());
                 }
-                returnedAliasNames.add(aliasMetadata.alias());
+                if (requestedAliases.length > 0) {
+                    for (final AliasMetadata aliasMetadata : cursor.getValue()) {
+                        returnedAliasNames.add(aliasMetadata.alias());
+                    }
+                }
             }
         }
-        dataStreamAliases.entrySet()
-            .stream()
-            .flatMap(entry -> entry.getValue().stream())
-            .forEach(dataStreamAlias -> returnedAliasNames.add(dataStreamAlias.getName()));
+        if (requestedAliases.length > 0) {
+            dataStreamAliases.entrySet()
+                .stream()
+                .flatMap(entry -> entry.getValue().stream())
+                .forEach(dataStreamAlias -> returnedAliasNames.add(dataStreamAlias.getName()));
+        }
 
         // compute explicitly requested aliases that have are not returned in the result
         final SortedSet<String> missingAliases = new TreeSet<>();