Search notes:

sys.dm_os_performance_counters

The column instance_name refers to a database rather than an instance.
The sum of the values of all databases can be queried by selecting with instance_name = '_Total': 
select
   *
from
   sys.dm_os_performance_counters with(nolock)
where
   instance_name = '_Total'

See also

The sys schema

Index

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