fix(router): check outer event key in failuresMetric guard to prevent inner-map wipe

[harsh4vardhan]

Summary

Resolves #7260.

The guard that initialises failuresMetric's inner map checks whether
the error string exists in the inner map, not whether the event
type key exists in the outer map:

// Before (checks inner key — wrong)
if _, ok := rt.telemetry.failuresMetric[event][errorStr]; !ok {
    rt.telemetry.failuresMetric[event] = make(map[string]int)
}

Whenever a destination produces a second distinct error string within
one reporting window, the inner-map lookup returns false (the new
string isn't there yet), make() fires, and every count accumulated
so far for that event type is lost. In a busy destination this resets
on every new error variant.

Fix

Check the outer event-type key so the inner map is initialised once
per event type and never overwritten:

if _, ok := rt.telemetry.failuresMetric[event]; !ok {
    rt.telemetry.failuresMetric[event] = make(map[string]int)
}

harsh4vardhan