feat: flow state size (#193)

* feat: add flow state size

* chore: protoc

* refactor: use FlowId not uint64

* fix: use uint32

* chore: make

proto/greptime/v1/meta/heartbeat.proto

@@ -47,6 +47,7 @@ message HeartbeatRequest {
   // The node's epoch
   uint64 node_epoch = 7;
   NodeInfo info = 8;
+  FlowStat flow_stat = 9;
 }
 
 enum RegionRole {
@@ -54,9 +55,11 @@ enum RegionRole {
   Leader = 0;
   // Readonly region.
   Follower = 1;
-  // A downgrading region, which is in the process of downgrading from Leader to Follower.
-  // 
-  // This role is used to prevent the region from being written during the downgrade process.
+  // A downgrading region, which is in the process of downgrading from Leader to
+  // Follower.
+  //
+  // This role is used to prevent the region from being written during the
+  // downgrade process.
   DowngradingLeader = 2;
 }
 
@@ -84,6 +87,13 @@ message RegionStat {
   map<string, bytes> extensions = 99;
 }
 
+message FlowStat {
+  // Each flow's in mem state's size in bytes
+  // due to protobuf's key can't be a message, so we use uint32 as the key which
+  // is [`FlowId`]'s inner field `id`
+  map<uint32, uint64> flow_stat_size = 1;
+}
+
 message HeartbeatResponse {
   ResponseHeader header = 1;
   MailboxMessage mailbox_message = 2;