feat(metrics): expose PD pipeline queue and HTTP send latency

[WANDY666]

Add separate queue-residence metrics for P/D forwarding, master ingress and request consumption, plus PD-master HTTP body-send duration and size. Queue samples use the oldest pending item; HTTP samples report window maxima, in seconds and bytes.

These distinguish model latency from forwarding, event-loop backlog and client backpressure. Generation time is excluded from send timing, normal services are unaffected, and both delayed and immediate response-header modes are preserved.

Tests: queue ages, metric registration, send boundaries and existing PD regressions (24 passed).