Do not serialize common stats flags using ordinal (#29600)

This commit remove serializing of common stats flags via its enum
ordinal and uses an explicit index defined on the enum. This is to
enable us to remove an unused flag (Suggest) without ruining the
ordering and thus breaking serialization.

server/src/main/java/org/elasticsearch/action/admin/indices/stats/CommonStatsFlags.java

@@ -53,7 +53,7 @@ public CommonStatsFlags(StreamInput in) throws IOException {
         final long longFlags = in.readLong();
         flags.clear();
         for (Flag flag : Flag.values()) {
-            if ((longFlags & (1 << flag.ordinal())) != 0) {
+            if ((longFlags & (1 << flag.getIndex())) != 0) {
                 flags.add(flag);
             }
         }
@@ -68,7 +68,7 @@ public CommonStatsFlags(StreamInput in) throws IOException {
     public void writeTo(StreamOutput out) throws IOException {
         long longFlags = 0;
         for (Flag flag : flags) {
-            longFlags |= (1 << flag.ordinal());
+            longFlags |= (1 << flag.getIndex());
         }
         out.writeLong(longFlags);
 
@@ -207,34 +207,39 @@ public CommonStatsFlags clone() {
     }
 
     public enum Flag {
-        // Do not change the order of these flags we use
-        // the ordinal for encoding! Only append to the end!
-        Store("store"),
-        Indexing("indexing"),
-        Get("get"),
-        Search("search"),
-        Merge("merge"),
-        Flush("flush"),
-        Refresh("refresh"),
-        QueryCache("query_cache"),
-        FieldData("fielddata"),
-        Docs("docs"),
-        Warmer("warmer"),
-        Completion("completion"),
-        Segments("segments"),
-        Translog("translog"),
-        Suggest("suggest"), // unused
-        RequestCache("request_cache"),
-        Recovery("recovery");
+        Store("store", 0),
+        Indexing("indexing", 1),
+        Get("get", 2),
+        Search("search", 3),
+        Merge("merge", 4),
+        Flush("flush", 5),
+        Refresh("refresh", 6),
+        QueryCache("query_cache", 7),
+        FieldData("fielddata", 8),
+        Docs("docs", 9),
+        Warmer("warmer", 10),
+        Completion("completion", 11),
+        Segments("segments", 12),
+        Translog("translog", 13),
+        Suggest("suggest", 14), // unused
+        RequestCache("request_cache", 15),
+        Recovery("recovery", 16);
 
         private final String restName;
+        private final int index;
 
-        Flag(String restName) {
+        Flag(final String restName, final int index) {
             this.restName = restName;
+            this.index = index;
         }
 
         public String getRestName() {
             return restName;
         }
+
+        private int getIndex() {
+            return index;
+        }
+
     }
 }