You probably don't have the same Magnolia version as blog's article.
And it seems that your javamelody configuration in web.xml is mixed with a
info.magnolia.cms.filters.MgnlMainFilter which was supposed to be already in web.xml.
I suggest to add in your web.xml something like the following, without adding a magnolia filter :
Then the monitoring page will be available at .../monitoring in your webapp, accorging to the monitoring-path parameter.
I recommend to never use a password which is the same as the username, even in development, because a simple password is sometimes put in production without changing it. It's better to use a randonly generated password instead.