HI there!
I've seen this post, also this github issue. Indeed, I'm able to sort by a custom metric, but only the first one. Take a look at this:
First of all, the metric column is not shown in the list of the selectable columns to sort, using the context menu. I need to click over the column in order to do so. All of those columns in the image are custom metrics. When I order the sorting by the first column, it works just fine. If I try to sort by any other metric column, it will do the sema - sort by the first column, no matter what column did I actually select.
Some context:
Browser: Chrome 67.0.3396.99 64 bits
OS: Windows 10
DB: MySQL
Metabase version: 0.29
Hostin env: Debian GNU/Linux 8, on EC2 server powered by Amazon
Metabase internal DB: H2
Steps to reproduce: create two diferent custom metrics; start a question, select them generating more than one row; sort by the second one - you should find out that the sorting was actually made by the first one.