Generates a String representation to identify a single instance of StreamProcessor.
This value can be representative of its current executing environment. It can also be custom-managed by the user,
as long as it adheres to the specification below. More than one processor can co-exist within the same JVM,
as long as their identifiers are guaranteed to be unique.
Specification of processor identifier:
Processor identifier has to be unique among the processors within a job
When more than one processor co-exist within the same JVM, the processor identifier can be of the format:
$x_$y, where 'x' is a unique identifier for the executing JVM and 'y' is a unique identifier for the
processor instance within the JVM. When there is only one processor within a JVM, 'x' should be sufficient to
uniquely identify the processor instance.
In case of more than one processors within the same JVM, the custom implementation of ProcessorIdGenerator can
contain a static counter, which is incremented on each call to generateProcessorId. The counter value can
be treated as the identifier for the processor instance within the JVM.