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

internal/debug/flags.go

@@ -95,7 +95,10 @@ var Flags = []cli.Flag{
 	memprofilerateFlag, blockprofilerateFlag, cpuprofileFlag, traceFlag,
 }
 
-var glogger *log.GlogHandler
+var (
+	ostream log.Handler
+	glogger *log.GlogHandler
+)
 
 func init() {
 	usecolor := term.IsTty(os.Stderr.Fd()) && os.Getenv("TERM") != "dumb"
@@ -103,14 +106,26 @@ func init() {
 	if usecolor {
 		output = colorable.NewColorableStderr()
 	}
-	glogger = log.NewGlogHandler(log.StreamHandler(output, log.TerminalFormat(usecolor)))
+	ostream = log.StreamHandler(output, log.TerminalFormat(usecolor))
+	glogger = log.NewGlogHandler(ostream)
 }
 
 // Setup initializes profiling and logging based on the CLI flags.
 // It should be called as early as possible in the program.
-func Setup(ctx *cli.Context) error {
+func Setup(ctx *cli.Context, logdir string) error {
 	// logging
 	log.PrintOrigins(ctx.GlobalBool(debugFlag.Name))
+	if logdir != "" {
+		rfh, err := log.RotatingFileHandler(
+			logdir,
+			262144,
+			log.JSONFormatOrderedEx(false, true),
+		)
+		if err != nil {
+			return err
+		}
+		glogger.SetHandler(log.MultiHandler(ostream, rfh))
+	}
 	glogger.Verbosity(log.Lvl(ctx.GlobalInt(verbosityFlag.Name)))
 	glogger.Vmodule(ctx.GlobalString(vmoduleFlag.Name))
 	glogger.BacktraceAt(ctx.GlobalString(backtraceAtFlag.Name))