observability: rename OperationMetrics to REDMetrics
Created by: Strum355
To clarify purpose/intent, this PR renames OperationMetrics and related types/methods/functions to REDMetrics. Functionally nothing changes, this simply came about due to my repeated confusion about what OperationMetrics does in comparison to other similarly named types, making it immediately clear from the name that it configures the RED triplet of metrics as per https://thenewstack.io/monitoring-microservices-red-method/