Add "inserted" metrics to processorhelper

[djaglowski]

Is your feature request related to a problem? Please describe.
The current set of autogenerated metrics in processorhelper do not represent telemetry which was inserted by a processor. This is not a terribly common use case but is technically possible and can be important to understand.

A couple examples:

• metrics generation processor
• An aggregation processor may aggregate 10 items into 1. When doing so, I think it would be reasonable to describe this as 10 dropped, 1 inserted.
• A delta-to-cumulative processor should arguably report N dropped, N inserted.

Describe the solution you'd like
I'd like to add one additional metric for each data type, called processor_inserted_*.