How to install Certbot plugins?
Better method, thanks to the others responses for helping me get to this.

Determine what plugins are installed currently:

# certbot-auto plugins
Saving debug log to /var/log/letsencrypt/letsencrypt.log

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* apache
Description: Apache Web Server plugin - Beta
Interfaces: IAuthenticator, IInstaller, IPlugin
Entry point: apache = certbot_apache.entrypoint:ENTRYPOINT

* nginx
Description: Nginx Web Server plugin
Interfaces: IAuthenticator, IInstaller, IPlugin
Entry point: nginx = certbot_nginx.configurator:NginxConfigurator

* standalone
Description: Spin up a temporary webserver
Interfaces: IAuthenticator, IPlugin
Entry point: standalone = certbot.plugins.standalone:Authenticator

* webroot
Description: Place files in webroot directory
Interfaces: IAuthenticator, IPlugin
Entry point: webroot = certbot.plugins.webroot:Authenticator
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Determine where your certbot (in my case certbot-auto) is installed:

# find / -name certbot
/opt/eff.org/certbot
...


Get into the Virtual Env and install plugin

cd /opt/eff.org/certbot/venv
source bin/activate
pip install certbot-dns-google
deactivate


Verify certbot plugins again

# certbot-auto plugins
Saving debug log to /var/log/letsencrypt/letsencrypt.log

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* apache
Description: Apache Web Server plugin - Beta
Interfaces: IAuthenticator, IInstaller, IPlugin
Entry point: apache = certbot_apache.entrypoint:ENTRYPOINT

* dns-google
Description: Obtain certificates using a DNS TXT record (if you are using Google
Cloud DNS for DNS).
Interfaces: IAuthenticator, IPlugin
Entry point: dns-google = certbot_dns_google.dns_google:Authenticator

* nginx
Description: Nginx Web Server plugin
Interfaces: IAuthenticator, IInstaller, IPlugin
Entry point: nginx = certbot_nginx.configurator:NginxConfigurator

* standalone
Description: Spin up a temporary webserver
Interfaces: IAuthenticator, IPlugin
Entry point: standalone = certbot.plugins.standalone:Authenticator

* webroot
Description: Place files in webroot directory
Interfaces: IAuthenticator, IPlugin
Entry point: webroot = certbot.plugins.webroot:Authenticator
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