Module http_handler_metrics 

Prometheus metrics for HTTP admission and synchronous execution.

Exposes the legacy handler-envelope series plus execution-admission gauges and queue-wait histograms per ADR-0017 (Prometheus / Grafana adapters), so operators can distinguish front-door pressure from engine saturation:

• http_active_handler_threads{transport} — legacy-named gauge of admitted requests, sourced from HttpConnectionLimiter::current() at scrape time.
• http_handler_cap{transport} — static gauge, sourced from the limiter’s cap. Same value for http and https since slice 3 collapsed both transports onto one cap.
• http_handler_rejected_total{transport, reason} — counter, incremented per admission or deadline rejection.
• http_handler_duration_seconds{transport} — histogram of total handler wall-clock time, sampled on every handler exit (happy-path and timeout). Buckets are the standard Prometheus client default set so existing dashboards render quantiles via histogram_quantile without configuration.
• http_execution_{workers,active,queued,queue_capacity} — live execution lane state.
• http_execution_queue_wait_seconds{transport} — time admitted requests spend waiting for a synchronous execution slot.

Counters and histogram updates are plain AtomicU64 operations on the hot path. No registry, no mutex.

Structs

HttpHandlerMetrics

Enums

HttpRejectReason

HttpTransport