feat: introduce a new task code for replicas stat (#2169)

Currently calculating stats for replicas is still controlled by a configuration of
`gc_disabled` and based on the  task code of  "garbage collect", which is already
the legacy for GC of shared logs. We could introduce individual controller and
task code for the replicas stats.

Configurations are changed as follows:

```diff
[replication]
+ replicas_stat_disabled = false
- gc_disabled = false
```

src/common/replication.codes.h

@@ -199,6 +199,7 @@ MAKE_EVENT_CODE(LPC_OPEN_REPLICA, TASK_PRIORITY_COMMON)
 MAKE_EVENT_CODE(LPC_CLOSE_REPLICA, TASK_PRIORITY_COMMON)
 MAKE_EVENT_CODE(LPC_CHECKPOINT_REPLICA, TASK_PRIORITY_COMMON)
 MAKE_EVENT_CODE(LPC_CATCHUP_WITH_PRIVATE_LOGS, TASK_PRIORITY_COMMON)
+MAKE_EVENT_CODE(LPC_REPLICAS_STAT, TASK_PRIORITY_COMMON)
 MAKE_EVENT_CODE(LPC_DISK_STAT, TASK_PRIORITY_COMMON)
 MAKE_EVENT_CODE(LPC_BACKGROUND_COLD_BACKUP, TASK_PRIORITY_COMMON)
 MAKE_EVENT_CODE(LPC_PARTITION_SPLIT_ASYNC_LEARN, TASK_PRIORITY_COMMON)

src/replica/replica_stub.cpp

@@ -276,11 +276,6 @@ DSN_DEFINE_bool(replication,
                 mem_release_enabled,
                 true,
                 "whether to enable periodic memory release");
-DSN_DEFINE_bool(
-    replication,
-    gc_disabled,
-    false,
-    "Whether to disable replica statistics. The name contains 'gc' is for legacy reason");
 DSN_DEFINE_bool(replication, disk_stat_disabled, false, "whether to disable disk stat");
 DSN_DEFINE_bool(
     replication,
@@ -341,6 +336,8 @@ bool check_mem_release_max_reserved_mem_percentage(int32_t value)
 DSN_DEFINE_validator(mem_release_max_reserved_mem_percentage,
                      &check_mem_release_max_reserved_mem_percentage);
 
+DSN_DEFINE_bool(replication, replicas_stat_disabled, false, "whether to disable replicas stat");
+
 DSN_DEFINE_uint32(replication,
                   replicas_stat_interval_ms,
                   30000,
@@ -630,9 +627,9 @@ void replica_stub::initialize(const replication_options &opts, bool clear /* = f
     }
 
     // replicas stat
-    if (!FLAGS_gc_disabled) {
+    if (!FLAGS_replicas_stat_disabled) {
         _replicas_stat_timer_task = tasking::enqueue_timer(
-            LPC_GARBAGE_COLLECT_LOGS_AND_REPLICAS,
+            LPC_REPLICAS_STAT,
             &_tracker,
             [this] { on_replicas_stat(); },
             std::chrono::milliseconds(FLAGS_replicas_stat_interval_ms),

src/replica/replica_stub.h

@@ -184,7 +184,6 @@ class replica_stub : public serverlet<replica_stub>, public ref_counter
     //
     void on_meta_server_connected();
     void on_meta_server_disconnected();
-    void on_replicas_stat();
     void on_disk_stat();
 
     //
@@ -385,6 +384,8 @@ class replica_stub : public serverlet<replica_stub>, public ref_counter
     };
     using replica_stat_info_by_gpid = std::unordered_map<gpid, replica_stat_info>;
 
+    void on_replicas_stat();
+
     void response_client(gpid id,
                          bool is_read,
                          dsn::message_ex *request,