Google does this automatically; it's not based on any particular "micro data" or tags in the html code. As far as I know, Google just looks at the content on your forum. This makes for a very impressive trick!
This feature was described on the Google blog several years ago:
Today, we introduced a new search feature that makes it easier for you
to find forum posts or discussions related to what you’re searching
for. This new addition to Google search results applies to sites that
tend to have a large number of posts on a specific topic. When several
different discussions on a site are relevant to your query, we indent
them under the primary result and include the date of each post.
The biggest problem is that often times the post/author count is out of date, even after the thread has been archived for a while. Google doesn't disclose how it crawls the data so it's possible that adding in some microdata could improve the accuracy of the results.
We theorized that adding a thread's page number to the html title on each page in a given thread would help Google but results were inconclusive. Read more on this experiment: http://www.ninjapost.com/blog/improving-seo-for-forum-search-results/)