Fix metric imports: use module-level access instead of name imports

Type-annotated metric variables (e.g. `geojson_cache_operations: Counter`)
don't exist as importable names until init_metrics() runs.  Switch all
`from api.metrics import <metric>` to `import api.metrics as m` and
access instruments as attributes at runtime to avoid ImportError.

rec/throttle_detector.py

@@ -159,8 +159,8 @@ def reset_throttle_metrics() -> None:
 def _increment_throttle_metric(event_type: str) -> None:
     """Safely increment the OTel throttle counter if metrics are initialised."""
     try:
-        from api.metrics import throttle_events_total
-        throttle_events_total.add(1, {"type": event_type})
+        import api.metrics as m
+        m.throttle_events_total.add(1, {"type": event_type})
     except Exception:
         pass  # Metrics not yet initialised (e.g. during tests)