Hi,
I don't know what is the threshold before such errors are considered a problem... the less the better I would say. So I. cannot help you too much for this part of the problem.
What I can add is that according to the graphics the errors should not be causing slow performances to users as they are very fews (and occurring in burts). Generally slow performance experienced by users is an indication of packet drops occurring along the path (in the Metro cloud maybe).
The best thing you can do is to setup some measuring tool to see if and how much traffic gets dropped between sites. If you dont have professional tools
you can use Iperf and install it on 2 PCs (one per site) and check stats during peak times (when normally users complain). If you see drops on Iperf but not on the input/output ports of the Metro devices you manage you might ask your ISP to investigate them.
Good Iperf tutorial is http://openmaniak.com/iperf.php
PS: Usually when you have Fast ports which are limited by the ISP to a lower value (via policing or shaping probably) you should start investigating there, as likely you have drops when traffic is bursty (and TCP traffic usually IS bursty).
Riccardo