Add docs for CCR stats API (#35439)

This commit adds API docs for the CCR stats endpoint.

docs/reference/ccr/apis/follow/get-follow-stats.asciidoc

@@ -190,6 +190,7 @@ GET /follower_index/_ccr/stats
 // CONSOLE
 
 The API returns the following results:
+
 [source,js]
 --------------------------------------------------
 {

docs/reference/ccr/apis/get-ccr-stats.asciidoc

@@ -12,12 +12,155 @@ Get {ccr} stats.
 
 ==== Description
 
-This API gets {ccr} stats.
+This API gets {ccr} stats. This API will return all stats related to {ccr}. In
+particular, this API returns stats about auto-following, and returns the same
+shard-level stats as in the <<ccr-get-follow-stats,get follower stats API>>.
 
 ==== Request
 
+//////////////////////////
+
+[source,js]
+--------------------------------------------------
+PUT /follower_index/_ccr/follow
+{
+  "remote_cluster" : "remote_cluster",
+  "leader_index" : "leader_index"
+}
+--------------------------------------------------
+// CONSOLE
+// TESTSETUP
+// TEST[setup:remote_cluster_and_leader_index]
+
+[source,js]
+--------------------------------------------------
+POST /follower_index/_ccr/pause_follow
+--------------------------------------------------
+// CONSOLE
+// TEARDOWN
+
+//////////////////////////
+
+[source,js]
+--------------------------------------------------
+GET /_ccr/stats
+--------------------------------------------------
+// CONSOLE
+
+==== Results
+
+This API returns the following information:
+
+`auto_follow_stats`::
+  (object) an object representing stats for the auto-follow coordinator
+
+This object consists of the following fields:
+
+`auto_follow_stats.number_of_failed_follow_indices`::
+  (long) the number of indices that the auto-follow coordinator failed to
+  automatically follow; the causes of recent failures are captured in the logs
+  of the elected master node, and in the
+  `auto_follow_stats.recent_auto_follow_errors` field
+
+`auto_follow_stats.number_of_failed_remote_cluster_state_requests`::
+  (long) the number of times that the auto-follow coordinator failed to retrieve
+  the cluster state from a remote cluster registered in a collection of
+  auto-follow patterns
+
+`auto_follow_stats.number_of_successful_follow_indices`::
+  (long) the number of indices that the auto-follow coordinator successfully
+  followed
+
+`auto_follow_stats.recent_auto_follow_errors`::
+  (array) an array of objects representing failures by the auto-follow
+  coordinator
+
+`follow_stats`::
+  (object) an object representing shard-level stats for follower indices; refer
+  to the details of the response in the
+  <<ccr-get-follow-stats,get follower stats API>>
+
+==== Example
+
+This example retrieves {ccr} stats:
+
 [source,js]
 --------------------------------------------------
 GET /_ccr/stats
 --------------------------------------------------
 // CONSOLE
+
+The API returns the following results:
+
+[source,js]
+--------------------------------------------------
+{
+  "auto_follow_stats" : {
+    "number_of_failed_follow_indices" : 0,
+    "number_of_failed_remote_cluster_state_requests" : 0,
+    "number_of_successful_follow_indices" : 1,
+    "recent_auto_follow_errors" : [ ]
+  },
+  "follow_stats" : {
+    "indices" : [
+      {
+        "index" : "follower_index",
+        "shards" : [
+          {
+            "remote_cluster" : "remote_cluster",
+            "leader_index" : "leader_index",
+            "follower_index" : "follower_index",
+            "shard_id" : 0,
+            "leader_global_checkpoint" : 1024,
+            "leader_max_seq_no" : 1536,
+            "follower_global_checkpoint" : 768,
+            "follower_max_seq_no" : 896,
+            "last_requested_seq_no" : 897,
+            "outstanding_read_requests" : 8,
+            "outstanding_write_requests" : 2,
+            "write_buffer_operation_count" : 64,
+            "follower_mapping_version" : 4,
+            "follower_settings_version" : 2,
+            "total_read_time_millis" : 32768,
+            "total_read_remote_exec_time_millis" : 16384,
+            "successful_read_requests" : 32,
+            "failed_read_requests" : 0,
+            "operations_read" : 896,
+            "bytes_read" : 32768,
+            "total_write_time_millis" : 16384,
+            "write_buffer_size_in_bytes" : 1536,
+            "successful_write_requests" : 16,
+            "failed_write_requests" : 0,
+            "operations_written" : 832,
+            "read_exceptions" : [ ],
+            "time_since_last_read_millis" : 8
+          }
+        ]
+      }
+    ]
+  }
+}
+--------------------------------------------------
+// TESTRESPONSE[s/"number_of_successful_follow_indices" : 1/"number_of_successful_follow_indices" : $body.auto_follow_stats.number_of_successful_follow_indices/]
+// TESTRESPONSE[s/"leader_global_checkpoint" : 1024/"leader_global_checkpoint" : $body.follow_stats.indices.0.shards.0.leader_global_checkpoint/]
+// TESTRESPONSE[s/"leader_max_seq_no" : 1536/"leader_max_seq_no" : $body.follow_stats.indices.0.shards.0.leader_max_seq_no/]
+// TESTRESPONSE[s/"follower_global_checkpoint" : 768/"follower_global_checkpoint" : $body.follow_stats.indices.0.shards.0.follower_global_checkpoint/]
+// TESTRESPONSE[s/"follower_max_seq_no" : 896/"follower_max_seq_no" : $body.follow_stats.indices.0.shards.0.follower_max_seq_no/]
+// TESTRESPONSE[s/"last_requested_seq_no" : 897/"last_requested_seq_no" : $body.follow_stats.indices.0.shards.0.last_requested_seq_no/]
+// TESTRESPONSE[s/"outstanding_read_requests" : 8/"outstanding_read_requests" : $body.follow_stats.indices.0.shards.0.outstanding_read_requests/]
+// TESTRESPONSE[s/"outstanding_write_requests" : 2/"outstanding_write_requests" : $body.follow_stats.indices.0.shards.0.outstanding_write_requests/]
+// TESTRESPONSE[s/"write_buffer_operation_count" : 64/"write_buffer_operation_count" : $body.follow_stats.indices.0.shards.0.write_buffer_operation_count/]
+// TESTRESPONSE[s/"follower_mapping_version" : 4/"follower_mapping_version" : $body.follow_stats.indices.0.shards.0.follower_mapping_version/]
+// TESTRESPONSE[s/"follower_settings_version" : 2/"follower_settings_version" : $body.follow_stats.indices.0.shards.0.follower_settings_version/]
+// TESTRESPONSE[s/"total_read_time_millis" : 32768/"total_read_time_millis" : $body.follow_stats.indices.0.shards.0.total_read_time_millis/]
+// TESTRESPONSE[s/"total_read_remote_exec_time_millis" : 16384/"total_read_remote_exec_time_millis" : $body.follow_stats.indices.0.shards.0.total_read_remote_exec_time_millis/]
+// TESTRESPONSE[s/"successful_read_requests" : 32/"successful_read_requests" : $body.follow_stats.indices.0.shards.0.successful_read_requests/]
+// TESTRESPONSE[s/"failed_read_requests" : 0/"failed_read_requests" : $body.follow_stats.indices.0.shards.0.failed_read_requests/]
+// TESTRESPONSE[s/"operations_read" : 896/"operations_read" : $body.follow_stats.indices.0.shards.0.operations_read/]
+// TESTRESPONSE[s/"bytes_read" : 32768/"bytes_read" : $body.follow_stats.indices.0.shards.0.bytes_read/]
+// TESTRESPONSE[s/"total_write_time_millis" : 16384/"total_write_time_millis" : $body.follow_stats.indices.0.shards.0.total_write_time_millis/]
+// TESTRESPONSE[s/"write_buffer_size_in_bytes" : 1536/"write_buffer_size_in_bytes" : $body.follow_stats.indices.0.shards.0.write_buffer_size_in_bytes/]
+// TESTRESPONSE[s/"successful_write_requests" : 16/"successful_write_requests" : $body.follow_stats.indices.0.shards.0.successful_write_requests/]
+// TESTRESPONSE[s/"failed_write_requests" : 0/"failed_write_requests" : $body.follow_stats.indices.0.shards.0.failed_write_requests/]
+// TESTRESPONSE[s/"operations_written" : 832/"operations_written" : $body.follow_stats.indices.0.shards.0.operations_written/]
+// TESTRESPONSE[s/"time_since_last_read_millis" : 8/"time_since_last_read_millis" : $body.follow_stats.indices.0.shards.0.time_since_last_read_millis/]