First, this doesn't work easily on a Raspberry Pi, which runs their own version of Linux.  (I'll be playing with that next.  The problem is you need MongoDB version 2.6 or later, and the Raspberry Pi version of Linux installs version 2.4.  Thus, you need to compile your own MongoDB version from source.  Fortunately, source is available, since it's open-source, but compiling your own usually involves also including/compiling/downloading a bunch of libraries, etc., so it can get involved.)  This is under Ubuntu Linux and I was running version 20.x.  Basically, you just follow the instructions here:

https://github.com/schmitzn/howto-nightscout-linux

except that where it mentions port 80, you must use port 443.  You must use https to connect, which means you need a certificate.  Everything else seems pretty straight-forward and I have verified that it works, pulling numbers from Dexcom share, if you configure BRIDGE_USER_NAME, BRIDGE_PASSWORD, and "bridge" under the Enable options.  Everywhere pages mention changing the "Config Vars", this maps directly to the environment variables in the "start.sh" file.

Now, on to trying to install my own MongoDB on a Raspberry Pi Zero W...