Minimum permissions for monitoring a Sql Server instance

I'm setting up Sql Monitor for the first time, and I was wondering if anyone could tell me the Sql permissions needed for monitoring a Sql Server instance? I would like to avoid using an account with SA access for the monitoring, but I'm not sure what access is actually needed.

I have been trying to figure this out through trial and error, but this is slow going. Is there some documentation around this (I haven't been able to find any)?

Thanks!

-Kevin