Ephemeral debug logfile (#16108)

**What type of PR is this?**
Feature

**What does this PR do? Why is it needed?**
This PR introduces an ephemeral log file that captures debug logs for 24
hours.

- it captures debug logs regardless of the user provided (or
non-provided) `--verbosity` flag.
- it allows a maximum of 250MB for each log file. 
- it keeps 1 backup logfile in case of size-based rotations. (as opposed
to time-based)
- this is enabled by default for beacon and validator nodes.
- the log files live in `datadir/logs/` directory under the names of
`beacon-chain.log` and `validator.log`. backups have a timestamp in
their name as well.
- the feature can be disabled using the `--disable-ephemeral-log-file`
flag.

cmd/validator/flags/flags.go

@@ -410,6 +410,12 @@ var (
 		Usage: "Maximum number of health checks to perform before exiting if not healthy. Set to 0 or a negative number for indefinite checks.",
 		Value: DefaultMaxHealthChecks,
 	}
+	// DisableEphemeralLogFile disables the 24 hour debug log file.
+	DisableEphemeralLogFile = &cli.BoolFlag{
+		Name:  "disable-ephemeral-log-file",
+		Usage: "Disables the creation of a debug log file that keeps 24 hours of logs.",
+		Value: false,
+	}
 )
 
 // DefaultValidatorDir returns OS-specific default validator directory.

cmd/validator/main.go

@@ -115,6 +115,7 @@ var appFlags = []cli.Flag{
 	debug.BlockProfileRateFlag,
 	debug.MutexProfileFractionFlag,
 	cmd.AcceptTosFlag,
+	flags.DisableEphemeralLogFile,
 }
 
 func init() {
@@ -199,6 +200,12 @@ func main() {
 				}
 			}
 
+			if !ctx.Bool(flags.DisableEphemeralLogFile.Name) {
+				if err := logs.ConfigureEphemeralLogFile(ctx.String(cmd.DataDirFlag.Name), ctx.App.Name); err != nil {
+					log.WithError(err).Error("Failed to configure debug log file")
+				}
+			}
+
 			// Fix data dir for Windows users.
 			outdatedDataDir := filepath.Join(file.HomeDir(), "AppData", "Roaming", "Eth2Validators")
 			currentDataDir := flags.DefaultValidatorDir()

cmd/validator/usage.go

@@ -75,6 +75,7 @@ var appHelpFlagGroups = []flagGroup{
 			cmd.GrpcMaxCallRecvMsgSizeFlag,
 			cmd.AcceptTosFlag,
 			cmd.ApiTimeoutFlag,
+			flags.DisableEphemeralLogFile,
 		},
 	},
 	{