fix(router): swap requestsMetric slice under exclusive Lock instead of RLock in collectMetrics

[harsh4vardhan]

Summary

Resolves #7261.

collectMetrics acquires requestsMetricLock.RLock() (shared) and
then sets rt.telemetry.requestsMetric = nil while holding it. Writing
shared state under RLock violates sync.RWMutex's contract.
trackRequestMetrics, called from every worker, takes the exclusive
Lock() to append to the same slice — a concurrent read+reset under
RLock races with those appends.

Fix

Take a short exclusive lock, swap the slice into a local snapshot,
release the lock, then iterate the snapshot outside the lock:

rt.telemetry.requestsMetricLock.Lock()
snapshot := rt.telemetry.requestsMetric
rt.telemetry.requestsMetric = nil
rt.telemetry.requestsMetricLock.Unlock()

for _, reqMetric := range snapshot { ... }

This is shorter under lock than the original (lock held only for the
pointer swap, not for the full iteration and diagnostics call) and
correct by sync.RWMutex contract.

harsh4vardhan