The report server cannot create the Report Requests performance counter. The application domain DefaultDomain failed to initialize. Error: Object reference not set to an instance of an object..Service

Reporting Services Upgrade failed (2008 to 2008 R2) due to a dll process was locked.

After performing reporting services upgrade from 2008 to 2008R2 SSRS service was not started
and it is showing the below error while bringing up the service.

The report server cannot create the Report Requests performance counter.
The application domain DefaultDomain failed to initialize. Error: Object reference not set to an instance of an object..Service

Generally this error will occur when reporting services performance dll file was locked by some process while performing the upgrade.

WorkAround
To repair the performance counters

1. Add the below path to PATH environment variable
path=C:\Windows\Microsoft.NET\Framework\v4.0.30319

2. Change the default directory to Reporting Services bin location
CD C:\Program Files\Microsoft SQL Server\MSRS10_50.MSSQLSERVER\Reporting Services\ReportServer\bin

Run the following command from the command prompt to uninstall the report server resources:
installutil.exe /u ReportingServicesLibrary.dll

Run the following command from the command prompt to re-install the report server resources:
installutil.exe ReportingServicesLibrary.dll