Convert more peer <--> peer REST calls (#19004)

* Convert more peer <--> peer REST calls
* Clean up in general.
* Add JSON wrapper.
* Add slice wrapper.
* Add option to make handler return nil error if no connection is given, `IgnoreNilConn`.

Converts the following:

```
+	HandlerGetMetrics
+	HandlerGetResourceMetrics
+	HandlerGetMemInfo
+	HandlerGetProcInfo
+	HandlerGetOSInfo
+	HandlerGetPartitions
+	HandlerGetNetInfo
+	HandlerGetCPUs
+	HandlerServerInfo
+	HandlerGetSysConfig
+	HandlerGetSysServices
+	HandlerGetSysErrors
+	HandlerGetAllBucketStats
+	HandlerGetBucketStats
+	HandlerGetSRMetrics
+	HandlerGetPeerMetrics
+	HandlerGetMetacacheListing
+	HandlerUpdateMetacacheListing
+	HandlerGetPeerBucketMetrics
+	HandlerStorageInfo
+	HandlerGetLocks
+	HandlerBackgroundHealStatus
+	HandlerGetLastDayTierStats
+	HandlerSignalService
+	HandlerGetBandwidth
```

cmd/admin-handlers.go

@@ -1984,7 +1984,6 @@ func (a adminAPIHandlers) TraceHandler(w http.ResponseWriter, r *http.Request) {
 // The ConsoleLogHandler handler sends console logs to the connected HTTP client.
 func (a adminAPIHandlers) ConsoleLogHandler(w http.ResponseWriter, r *http.Request) {
 	ctx := r.Context()
-
 	objectAPI, _ := validateAdminReq(ctx, w, r, policy.ConsoleLogAdminAction)
 	if objectAPI == nil {
 		return
@@ -2009,44 +2008,65 @@ func (a adminAPIHandlers) ConsoleLogHandler(w http.ResponseWriter, r *http.Reque
 
 	setEventStreamHeaders(w)
 
-	logCh := make(chan log.Info, 4000)
-
+	logCh := make(chan log.Info, 1000)
 	peers, _ := newPeerRestClients(globalEndpoints)
-
+	encodedCh := make(chan []byte, 1000+len(peers)*1000)
 	err = globalConsoleSys.Subscribe(logCh, ctx.Done(), node, limitLines, logKind, nil)
 	if err != nil {
 		writeErrorResponseJSON(ctx, w, toAdminAPIErr(ctx, err), r.URL)
 		return
 	}
+	// Convert local entries to JSON
+	go func() {
+		var buf bytes.Buffer
+		enc := json.NewEncoder(&buf)
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case li := <-logCh:
+				if !li.SendLog(node, logKind) {
+					continue
+				}
+				buf.Reset()
+				if err := enc.Encode(li); err != nil {
+					continue
+				}
+				select {
+				case <-ctx.Done():
+					return
+				case encodedCh <- append(grid.GetByteBuffer()[:0], buf.Bytes()...):
+				}
+			}
+		}
+	}()
 
+	// Collect from matching peers
 	for _, peer := range peers {
 		if peer == nil {
 			continue
 		}
 		if node == "" || strings.EqualFold(peer.host.Name, node) {
-			peer.ConsoleLog(logCh, ctx.Done())
+			peer.ConsoleLog(ctx, logKind, encodedCh)
 		}
 	}
 
-	enc := json.NewEncoder(w)
-
 	keepAliveTicker := time.NewTicker(500 * time.Millisecond)
 	defer keepAliveTicker.Stop()
-
 	for {
 		select {
-		case log, ok := <-logCh:
+		case log, ok := <-encodedCh:
 			if !ok {
 				return
 			}
-			if log.SendLog(node, logKind) {
-				if err := enc.Encode(log); err != nil {
-					return
-				}
-				if len(logCh) == 0 {
-					// Flush if nothing is queued
-					w.(http.Flusher).Flush()
-				}
+			_, err = w.Write(log)
+			if err != nil {
+				return
+			}
+			grid.PutByteBuffer(log)
+			if len(logCh) == 0 {
+				// Flush if nothing is queued
+				w.(http.Flusher).Flush()
 			}
 		case <-keepAliveTicker.C:
 			if len(logCh) > 0 {