1. A computer program product for implementing a computer implemented method for instrumenting performance counters in an application, the computer program product comprising one or more computer-readable storage devices having encoded thereon computer executable instructions which, when executed upon one or more processors within a computing system, perform the method for instrumenting performance counters in an application, the method comprising: loading an instrumentation configuration to determine one or more application methods requiring performance counter instrumentation and one or more types of performance counter that are to be instrumented for each such application method; locating a segment of a method body that is to be instrumented with said one or more types of performance counter; and modifying the segment of the method body to expose one or more types of performance counter, wherein said modifying step further comprises: inserting declarative instrumentation filtering code into said method body, said declarative instrumentation filtering code comprising computer executable instructions configured to: retrieve values from application method runtime parameters and class member variables; calculate filtering expressions; and determine if a method runtime context matches a filtering rule set associated with said one or more types of performance counters that are to be instrumented.