Everytime I greet my roomate with "How are you?" he actually tells me in detail how he is. Is that normal?

He will answer with something like "I'm fine, thanks. How about you?" or "Quite well. And yourself?" every so often. But most days he'll say something like "I feel a bit sleepy today. Last night was not so great..." or "I feel energized, I think I can do lots of things today...".

My wife, who is American, warns people who don’t know me not to ask how I'm doing, because I'll give them a "full report" on my life.

Don't ask how someone is feeling if you don't actually want to know. That kind of politeness is not necessary.

If you ask a German person how they are, they will give you a straightforward answer. It's that easy.

It's not that surprising to me as he is your roommate, not a random person.

Many Germans, even if they speak good English, may not realize that the phrase "How are you? Fine, and you? Good!" is a set sequence used as a greeting. They may think it is a genuine question and try to answer it out of politeness. However, this is more of a language problem than a cultural one. It can be explained to them that it is simply a greeting, and most people will stop answering it. The phrase is a "false friend", as it is a translation of each other on the vocabulary level, but not the semantic level. Naturally, this depends on the context, as in other situations "How are you?" may be a genuine question.

Germans don't engage in small talk; if you ask a question, you'll get a straightforward answer.

In English, "How are you" is a greeting, which translates to "Hello" in German. Whereas in German, the same phrase means "How are you".

You ask someone how they are and are surprised when they give an honest answer?

Are you looking for smalltalk? You can signal that by asking a closed question like "Alles klar...?" which suggests that you don't want a long discussion.