Add cluster section into the info command (#87)

This was added in apache/kvrocks#1379

docs/info-sections.md

@@ -13,6 +13,7 @@ The optional section parameter can be used to select a specific section of infor
 | replication  | Master/replica replication information       |
 | cpu          | CPU consumption statistics                   |
 | commandstats | Kvrocks command statistics                   |
+| cluster      | Cluster section                              |
 | keyspace     | Database related statistics                  |
 | rocksdb      | RocksDB related statistics                   |
 
@@ -75,6 +76,9 @@ used_cpu_user:0
 cmdstat_command:calls=1,usec=904,usec_per_call=904
 cmdstat_info:calls=1,usec=0,usec_per_call=0
 
+# Cluster
+cluster_enabled:0
+
 # Keyspace
 # Last scan db time: Thu Jan  1 08:00:00 1970
 db0:keys=0,expires=0,avg_ttl=0,expired=0
@@ -227,6 +231,14 @@ Here is the meaning of all fields in the cpu section:
 | used_cpu_sys  | System CPU consumed by the Kvrocks server, which is the sum of system CPU consumed by all threads of the server process (main thread and background threads) |
 | used_cpu_user | User CPU consumed by the Kvrocks server, which is the sum of user CPU consumed by all threads of the server process (main thread and background threads)     |
 
+## Cluster section
+
+Here is the meaning of all fields in the cluster section:
+
+| Property         | Desc                                                     |
+|------------------|----------------------------------------------------------|
+| cluster_enabled  | Indicate Kvrocks cluster is enabled                      |
+
 ## Keyspace section
 
 The keyspace section provides statistics on the main dictionary of each database. The statistics are the number of keys, and the number of keys with an expiration. Note that Kvrocks only have `db0` and keys statistics wasn't manipulated in memory, so we need to use the `dbsize scan` to async scan and calculate the keys number like below: