metrics: Prometheus exporter sample + worked INTEGRATION example

README §3 promised a monitoring story ("aggregate into Prometheus via
a sidecar that polls the data model").  Nothing shipped.  Customers
running a real fab without a metrics pipeline find out about T7
storms, spool blowups, and stalled CJs after their MES does — not
the position you want SRE in.

This commit ships:

- include/secsgem/metrics/prometheus.hpp: header-only.  A Registry
  (counters + gauges + HELP/TYPE descriptions, label-keyed,
  mutex-guarded so updates from the io thread and scrape renders from
  the same io serialize cleanly) plus a PrometheusServer (asio
  acceptor, replies to any GET with the text-exposition rendering,
  no auth — drop nginx in front for that).

- tests/test_metrics_prometheus.cpp: 3 cases / 19 assertions.
  Render counter+gauge with labels, scrape via raw TCP and parse the
  HTTP body, verify live updates land on subsequent scrapes.

- INTEGRATION.md §6.4: worked example that pairs the exporter with the
  Connection + EquipmentDataModel hooks documented in §6.1/§6.2.
  Shows the wrap-around-handler trick for message counters, a 5s
  polling timer for gauges (spool depth, active alarms), and the
  expected /metrics output.

Deliberately *not* shipped:
- A StandardMetrics helper that auto-wires everything — would force
  a single hook owner per store, breaking customers who want
  composable observers.  Customers wire what they need; the registry
  gives them counters + gauges + an HTTP endpoint, no policy.
- TLS / auth on the HTTP endpoint.  Reverse-proxy territory.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CMakeLists.txt

@@ -147,6 +147,7 @@ add_executable(secsgem_tests
   tests/test_thread_safety.cpp
   tests/test_persistence_upgrade.cpp
   tests/test_config_validate.cpp
+  tests/test_metrics_prometheus.cpp
 )
 target_link_libraries(secsgem_tests PRIVATE secsgem doctest::doctest)
 target_compile_definitions(secsgem_tests PRIVATE