A few weeks ago I wrote about gathering some Git repository metrics and viewing them in the Git MultiSite GUI or in Graphite, and someone pointed out that some of the administrative metrics useful for capacity planning could be gathered using monitoring tools like Nagios. Repository size on disk is a good example since system administrators normally monitor disk space to make sure that the server doesn’t run out of space. You can set up Nagios to monitor any file system that contains Git repositories.
This level of check_disk information is useful for high level monitoring, and there are many plugins to connect Nagios to rrd graphing tools. I used OpsView to set up my example which includes a built-in graphing capability.
Drilling down to individual repositories would be an easy modification to the check_disk plugin, or you can get more granular data from Git MultiSite.