1. A method to process application performance data, the method performed by at least one computer processor executing computer-readable instructions tangibly stored on at least one computer-readable medium, the method comprising the steps of: (A) injecting an agent into a monitored application; (B) at the agent: (B)(1) capturing original byte code of the monitored application; (B) (2) sending the captured byte code of the monitored application to a monitoring server external to the monitored application; (C) at an instrumentation engine in the monitoring server: (C) (1) inserting sensor bytecode into the captured original bytecode to generate instrumented byte code at runtime of the application and without altering source code of the application; (C) (2) transmitting the instrumented byte code to the monitored application; (D) at the monitored application: (D) (1) loading the instrumented byte code instead of the original bytecode; (D) (2) executing the sensor bytecode to generate measurement data which describes performance characteristics of the original bytecode; and (D) (3) writing the measurement data to a data set of fixed size.