perf(metarepos): add mrpb.StorageNodeUncommitReport pool

This patch adds an `mrpb.StorageNodeUncommitReport` pool, which is
`proto/mrpb.storageNodeUncommitReportPool`. It reduces heap objects allocated repeatedly whenever
processing reports received from storage nodes.

Below heap profiling is the previous result.

```
      flat  flat%   sum%        cum   cum%
  70865580  8.38% 44.73%   71293773  8.43%  github.com/kakao/varlog/internal/metarepos.(*reportCollectExecutor).processReport
```

Now here it is after applying this patch.

```
      flat  flat%   sum%        cum   cum%
  20734504  2.10% 75.92%   42122814  4.26%  github.com/kakao/varlog/internal/metarepos.(*reportCollectExecutor).processReport
```

internal/metarepos/report_collector.go

@@ -636,6 +636,7 @@ func (rce *reportCollectExecutor) getReport(ctx context.Context) error {
 	}
 
 	report := rce.processReport(response)
+	defer report.Release()
 	if report.Len() > 0 {
 		if err := rce.helper.ProposeReport(rce.storageNodeID, report.UncommitReports); err != nil {
 			rce.reportCtx.setExpire()
@@ -646,10 +647,8 @@ func (rce *reportCollectExecutor) getReport(ctx context.Context) error {
 }
 
 func (rce *reportCollectExecutor) processReport(response *snpb.GetReportResponse) *mrpb.StorageNodeUncommitReport {
-	report := &mrpb.StorageNodeUncommitReport{
-		StorageNodeID:   response.StorageNodeID,
-		UncommitReports: response.UncommitReports,
-	}
+	report := mrpb.NewStoragenodeUncommitReport(response.StorageNodeID)
+	report.UncommitReports = response.UncommitReports
 
 	if report.Len() == 0 {
 		return report
@@ -664,9 +663,9 @@ func (rce *reportCollectExecutor) processReport(response *snpb.GetReportResponse
 		return report
 	}
 
-	diff := &mrpb.StorageNodeUncommitReport{
-		StorageNodeID: report.StorageNodeID,
-	}
+	diff := mrpb.NewStoragenodeUncommitReport(report.StorageNodeID)
+	diff.UncommitReports = make([]snpb.LogStreamUncommitReport, 0, len(report.UncommitReports))
+	defer report.Release()
 
 	i := 0
 	j := 0
@@ -919,7 +918,8 @@ func (rc *reportContext) saveReport(report *mrpb.StorageNodeUncommitReport) {
 	rc.mu.Lock()
 	defer rc.mu.Unlock()
 
-	rc.report = report
+	rc.report = mrpb.NewStoragenodeUncommitReport(report.StorageNodeID)
+	rc.report.UncommitReports = report.UncommitReports
 }
 
 func (rc *reportContext) getReport() *mrpb.StorageNodeUncommitReport {

proto/mrpb/raft_metadata_repository.go

@@ -2,6 +2,7 @@ package mrpb
 
 import (
 	"sort"
+	"sync"
 
 	"github.com/kakao/varlog/pkg/types"
 	"github.com/kakao/varlog/proto/snpb"
@@ -75,6 +76,24 @@ func (l *LogStreamUncommitReports) Deleted() bool {
 	return l.Status == varlogpb.LogStreamStatusDeleted
 }
 
+var storageNodeUncommitReportPool = sync.Pool{
+	New: func() any {
+		return new(StorageNodeUncommitReport)
+	},
+}
+
+func NewStoragenodeUncommitReport(snid types.StorageNodeID) *StorageNodeUncommitReport {
+	r := storageNodeUncommitReportPool.Get().(*StorageNodeUncommitReport)
+	r.StorageNodeID = snid
+	return r
+}
+
+func (l *StorageNodeUncommitReport) Release() {
+	l.StorageNodeID = types.StorageNodeID(0)
+	l.UncommitReports = nil
+	storageNodeUncommitReportPool.Put(l)
+}
+
 func (l *StorageNodeUncommitReport) Len() int {
 	return len(l.UncommitReports)
 }