wrap `log` messages with `hclog` (#11291)

.changelog/11291.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+agent: Fixed an issue that caused some non-JSON log output when `log_json` was enabled
+```

command/agent/command.go

@@ -470,7 +470,6 @@ func SetupLoggers(ui cli.Ui, config *Config) (*logutils.LevelFilter, *gatedwrite
 	}
 
 	logOutput := io.MultiWriter(writers...)
-	log.SetOutput(logOutput)
 	return logFilter, logGate, logOutput
 }
 
@@ -649,6 +648,12 @@ func (c *Command) Run(args []string) int {
 		JSONFormat: config.LogJson,
 	})
 
+	// Wrap log messages emitted with the 'log' package.
+	// These usually come from external dependencies.
+	log.SetOutput(logger.StandardWriter(&hclog.StandardLoggerOptions{InferLevels: true}))
+	log.SetPrefix("")
+	log.SetFlags(0)
+
 	// Swap out UI implementation if json logging is enabled
 	if config.LogJson {
 		c.Ui = &logging.HcLogUI{Log: logger}