[FaultFilter] Inaccurate faults overflow metrics

[Augustyniak]

Intro

One of the first operations Envoy's fault filter performs when it receives an incoming network request is checking for whether the maximum number of active faults has been reached or not. If the number of currently active faults is greater than agreed on maximum limit, Envoy increments faults_overflow counter which is supposed to help its users to understand when too many flows are flowing into the system.

Issue

Envoy increases the value of faults_overflow counter without checking whether an incoming requests "contains" a fault or not. This results in faults_overflow metric being inaccurate. Whether faults_overflow metrics is incremented or not for cases when fault filter is serving the maximum number of faults, should depend on whether Envoy was instructed to inject a fault into a given incoming network request - the one being processed at the moment. If there is no fault to inject, the faults_overflow metric should not be incremented even if the number of active_faults is at its maximum value.