better find PerformanceCounterInstance

It does not always 'startswith' - running as application or debugging on my win81 pc the Instancename is:
'Raven.Server.vshost/1-UxBsU-C:\Users\SteinArild\Source\Repos\RavenDb\build\Database'
trying to match uniquePrefix = '1-UxBsU'

Found this because I have a 22 index database  where "MaybePulseTransactions()" never hit because of this - and I ended up with EsentOutOfMemory exceptions....