Captcha challenge for layer 7, or application-layer attack protection

As a web-hosting provider, I would like to have a Captcha challenge protection to prevent saturation of server resources so that to protect our web services from attacks on layer 7.

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For example, when the cPanel user selects to activate the protection in his domain, to install a front proxy that proposes to the visitors the resolution of Google captcha, if it resolves, we redirect the visitor to the web server, if it does not resolve, we redirect the user to the url=http://0.0.0.0

In order to configure the protection more selectively, this proxy could also be configurable to filter by country, refer, user-agent, etc...