TRT-LLM additional metric children are absent until their first event #18169

Problem

AdditionalMetricsCollector constructs labeled Prometheus families but does not create their children during initialization. A newly started TRT-LLM worker therefore omits supported counters such as trtllm_num_aborted_requests_total and trtllm_request_type_structured_output_total until each event first occurs. Dashboards and absent() checks cannot distinguish a supported zero from missing instrumentation.

Expected behavior

Create the already-known bounded label combination for each additional metric at collector startup. Keep all names, labels, event accounting, and engine pass-through metrics unchanged. A fresh registry should export zero-valued counters/gauges and zero-observation histograms before any request; subsequent events should accumulate normally.

This is a registration fix under the existing metrics guidelines (R1/L6), separate from cache-reuse accounting and new router funnel instrumentation.