Fetch the Mongo database name from the URI.

We were always using the `admin` database which is usually not what you want.

For example if your mongo uri is: `mongodb://user:pass@localhost/mydb` then `dbstats` will be called on the correct database.

So I don't know this check well enough to know if there is a particularly reason that we were using the `admin` database by default. I was able to call the other command (`serverStatus`) on any DB but this might be limited for other cases.