cmd, dashboard, log: log collection and exploration (#17097)

* cmd, dashboard, internal, log, node: logging feature

* cmd, dashboard, internal, log: requested changes

* dashboard, vendor: gofmt, govendor, use vendored file watcher

* dashboard, log: gofmt -s -w, goimports

* dashboard, log: gosimple

dashboard/message.go

@@ -16,7 +16,10 @@
 
 package dashboard
 
-import "time"
+import (
+	"encoding/json"
+	"time"
+)
 
 type Message struct {
 	General *GeneralMessage `json:"general,omitempty"`
@@ -67,6 +70,24 @@ type SystemMessage struct {
 	DiskWrite      ChartEntries `json:"diskWrite,omitempty"`
 }
 
+// LogsMessage wraps up a log chunk. If Source isn't present, the chunk is a stream chunk.
 type LogsMessage struct {
-	Log []string `json:"log,omitempty"`
+	Source *LogFile        `json:"source,omitempty"` // Attributes of the log file.
+	Chunk  json.RawMessage `json:"chunk"`            // Contains log records.
+}
+
+// LogFile contains the attributes of a log file.
+type LogFile struct {
+	Name string `json:"name"` // The name of the file.
+	Last bool   `json:"last"` // Denotes if the actual log file is the last one in the directory.
+}
+
+// Request represents the client request.
+type Request struct {
+	Logs *LogsRequest `json:"logs,omitempty"`
+}
+
+type LogsRequest struct {
+	Name string `json:"name"` // The request handler searches for log file based on this file name.
+	Past bool   `json:"past"` // Denotes whether the client wants the previous or the next file.
 }