Plugin that updates your custom version string every time you export the project. It can fetch the current version based on number of git commits or extract it from your export profile or you can configure it to use whatever you want.
It creates a script called version.gd, that contains a constant string named VERSION. You can use it like this: load(res://version.gd).VERSION. Put it anywhere and it will update automatically whenever you export, so you don't need to remember about it anymore. You can also store the version in your Project Settings instead of script.
Be sure to open the AutoExportVersion.gd script to configure the plugin before enabling it.
Check the repository page for more details. The plugin has also a built-in documentation.