The feature that has been within Google Webmaster Tools since June 28, 2011 is gone. Why?
Well, Google said they are removing it. They said in late September, that +1 Reports in Webmaster Tools is also going away on November 14th. They wrote:
We introduced +1 Reports in Webmaster Tools to help publishers measure +1 activity on their pages. Given that webmasters now use Social Reports in Google Analytics to get a wider view of social activity (including +1’s), we'll be discontinuing the stand-alone +1 Reports on November 14. Measuring social media remains a priority for Google Analytics, so stay tuned for future improvements.
So now if you login to your Google Webmaster Tools you will not see the +1 Metrics option.
Forum discussion at Google Webmaster Help.