Follow stats structure (#34301)

This commit modifies the follow stats API response structure to more
clearly highlight meaning of the higher level fields. In particular,
previously the response had a top-level key for each index. Instead, we
nest the indices under an "indices" field which is now an array. The
values in this array are objects containing two fields: "index" which is
the name of the follower index, and "shards" which is an array where
each value in the array is the follower stats for that shard. That is,
we have gone from:

{
  "bar": [
    {
      "shard_id": 0...
    }...
  ]...
}

to

{
  "indices": [
    {
      "index": "bar",
      "shards": [
        {
          "shard_id": 0...
        }...
      ]
   }...
}

Author: jasontedor

x-pack/plugin/ccr/qa/rest/src/test/resources/rest-api-spec/test/ccr/follow_stats.yml

@@ -0,0 +1,59 @@
+---
+"Test stats":
+  - do:
+      indices.create:
+        index: foo
+        body:
+          settings:
+            index:
+              soft_deletes:
+                enabled: true
+          mappings:
+            doc:
+              properties:
+                field:
+                  type: keyword
+
+  - do:
+      ccr.follow:
+        index: bar
+        body:
+          leader_index: foo
+  - is_true: follow_index_created
+  - is_true: follow_index_shards_acked
+  - is_true: index_following_started
+
+  # we can not reliably wait for replication to occur so we test the endpoint without indexing any documents
+  - do:
+      ccr.stats:
+        index: bar
+  - match: { indices.0.index: "bar" }
+  - match: { indices.0.shards.0.leader_index: "foo" }
+  - match: { indices.0.shards.0.follower_index: "bar" }
+  - match: { indices.0.shards.0.shard_id: 0 }
+  - gte: { indices.0.shards.0.leader_global_checkpoint: -1 }
+  - gte: { indices.0.shards.0.leader_max_seq_no: -1 }
+  - gte: { indices.0.shards.0.follower_global_checkpoint: -1 }
+  - gte: { indices.0.shards.0.follower_max_seq_no: -1 }
+  - gte: { indices.0.shards.0.last_requested_seq_no: -1 }
+  - gte: { indices.0.shards.0.number_of_concurrent_reads: 0 }
+  - match: { indices.0.shards.0.number_of_concurrent_writes: 0 }
+  - match: { indices.0.shards.0.number_of_queued_writes: 0 }
+  - gte: { indices.0.shards.0.mapping_version: 0 }
+  - gte: { indices.0.shards.0.total_fetch_time_millis: 0 }
+  - gte: { indices.0.shards.0.number_of_successful_fetches: 0 }
+  - gte: { indices.0.shards.0.number_of_failed_fetches: 0 }
+  - match: { indices.0.shards.0.operations_received: 0 }
+  - match: { indices.0.shards.0.total_transferred_bytes: 0 }
+  - match: { indices.0.shards.0.total_index_time_millis: 0 }
+  - match: { indices.0.shards.0.number_of_successful_bulk_operations: 0 }
+  - match: { indices.0.shards.0.number_of_failed_bulk_operations: 0 }
+  - match: { indices.0.shards.0.number_of_operations_indexed: 0 }
+  - length: { indices.0.shards.0.fetch_exceptions: 0 }
+  - gte: { indices.0.shards.0.time_since_last_fetch_millis: -1 }
+
+  - do:
+      ccr.pause_follow:
+        index: bar
+  - is_true: acknowledged
+

x-pack/plugin/ccr/qa/rest/src/test/resources/rest-api-spec/test/ccr/stats.yml

@@ -76,15 +76,24 @@ public XContentBuilder toXContent(final XContentBuilder builder, final Params pa
             }
             builder.startObject();
             {
-                for (final Map.Entry<String, Map<Integer, StatsResponse>> index : taskResponsesByIndex.entrySet()) {
-                    builder.startArray(index.getKey());
-                    {
-                        for (final Map.Entry<Integer, StatsResponse> shard : index.getValue().entrySet()) {
-                            shard.getValue().status().toXContent(builder, params);
+                builder.startArray("indices");
+                {
+                    for (final Map.Entry<String, Map<Integer, StatsResponse>> index : taskResponsesByIndex.entrySet()) {
+                        builder.startObject();
+                        {
+                            builder.field("index", index.getKey());
+                            builder.startArray("shards");
+                            {
+                                for (final Map.Entry<Integer, StatsResponse> shard : index.getValue().entrySet()) {
+                                    shard.getValue().status().toXContent(builder, params);
+                                }
+                            }
+                            builder.endArray();
                         }
+                        builder.endObject();
                     }
-                    builder.endArray();
                 }
+                builder.endArray();
             }
             builder.endObject();
             return builder;