Fix flag parsing on database plugins, exit on parse error (#3305)

* Change FlagSet error handling to flag.ExitOnError

* Ignore os.Args[0] (the command itself) when parsing flags

* Revert to using flag.ContinueOnError

plugins/database/mysql/mysql-legacy-database-plugin/main.go

@@ -11,7 +11,7 @@ import (
 func main() {
 	apiClientMeta := &pluginutil.APIClientMeta{}
 	flags := apiClientMeta.FlagSet()
-	flags.Parse(os.Args)
+	flags.Parse(os.Args[1:])
 
 	err := mysql.RunLegacy(apiClientMeta.GetTLSConfig())
 	if err != nil {