perf(metarepos): add a pool for mrpb.Reports

This change adds the pool for mrpb.Reports, which makes the metadata repository reuse heap object
for it.

internal/metarepos/raft_metadata_repository.go

@@ -306,6 +306,8 @@ Loop:
 				mr.logger.Error(err.Error())
 				continue
 			}
+			// Release *mrpb.Reports only if e contains Reports.
+			e.GetRequest().Report.Release()
 
 			select {
 			case mr.rnProposeC <- b:
@@ -347,10 +349,7 @@ func (mr *RaftMetadataRepository) processReport(ctx context.Context) {
 			mr.muReportQueue.Lock()
 			num := len(mr.reportQueue)
 			if num > 0 {
-				reports = &mrpb.Reports{
-					NodeID:      mr.nodeID,
-					CreatedTime: time.Now(),
-				}
+				reports = mrpb.NewReports(mr.nodeID, time.Now())
 				reports.Reports = mr.reportQueue
 				mr.reportQueue = make([]*mrpb.Report, 0, 1024)
 			}

proto/mrpb/raft_entry.go

@@ -0,0 +1,28 @@
+package mrpb
+
+import (
+	"sync"
+	"time"
+
+	"github.com/kakao/varlog/pkg/types"
+)
+
+var reportsPool = sync.Pool{
+	New: func() any {
+		return &Reports{}
+	},
+}
+
+func NewReports(nodeID types.NodeID, ts time.Time) *Reports {
+	rs := reportsPool.Get().(*Reports)
+	rs.NodeID = nodeID
+	rs.CreatedTime = ts
+	return rs
+}
+
+func (rs *Reports) Release() {
+	if rs != nil {
+		*rs = Reports{}
+		reportsPool.Put(rs)
+	}
+}