otel P3: auto-instrument handlers at the make-app seam

otel/instrument-routes wraps each flattened Dream route's handler in a timed
span named METHOD /route with {:http.method :http.route :http.status} attrs;
host/make-app applies it so every matched request becomes a trace. Refactored
with-span onto a shared otel/-timed core that takes a finalize fn for
result-derived attrs (the http.status only known post-handler).

lib/host/otel.sx

@@ -73,13 +73,15 @@
       (set! otel/-stack (list))
       nil)))
 
-;; ── with-span: the timed-effect combinator ───────────────────────────
+;; ── -timed: the shared timed-effect core ─────────────────────────────
 ;; Records a span around (thunk): a fresh span id, the trace inherited from the
 ;; enclosing span (or a new trace at the root), the enclosing span as :parent, and
-;; t0/t1 straddling the call. Pushes its context so nested with-spans see it as
-;; parent, pops after, then records the finished span. Returns the thunk's value.
-(define otel/with-span
-  (fn (name attrs thunk)
+;; t0/t1 straddling the call. Pushes its context so nested spans see it as parent,
+;; pops after, then records the finished span. `finalize` maps the thunk's result
+;; to EXTRA attrs merged onto `attrs` (e.g. an http.status only known post-call).
+;; Returns the thunk's value unchanged.
+(define otel/-timed
+  (fn (name attrs finalize thunk)
     (let ((parent-ctx (otel/-top)))
       (let ((trace  (if parent-ctx (get parent-ctx :trace) (otel/gen-trace-id)))
             (span   (otel/gen-span-id))
@@ -91,5 +93,36 @@
                 (otel/-pop!)
                 (otel/record!
                   {:trace trace :span span :parent parent :name name
-                   :t0 t0 :t1 t1 :attrs attrs :events (list)})
+                   :t0 t0 :t1 t1
+                   :attrs (merge attrs (finalize result))
+                   :events (list)})
                 result))))))))
+
+;; with-span — a plain timed span; no result-derived attrs.
+(define otel/with-span
+  (fn (name attrs thunk)
+    (otel/-timed name attrs (fn (r) {}) thunk)))
+
+;; ── auto-instrument HTTP handlers ─────────────────────────────────────
+;; A trace is a composition; a request handler is a timed effect. Wrap one Dream
+;; route's handler so dispatching it records a root span "METHOD /route" with
+;; {:http.method :http.route :http.status}. The status is only known after the
+;; handler runs, so it's a finalize attr; the result is coerced the same way the
+;; router coerces it, so a bare-string handler still reports 200. The raw handler
+;; result is returned untouched so dr/run-route's own coercion is unchanged.
+(define otel/instrument-route
+  (fn (r)
+    (let ((method  (dream-route-method r))
+          (path    (dream-route-path r))
+          (handler (dream-route-handler r)))
+      (dream-route method path
+        (fn (req)
+          (otel/-timed
+            (str method " " path)
+            {:http.method method :http.route path}
+            (fn (resp) {:http.status (dream-status (dream-coerce-response resp))})
+            (fn () (handler req))))))))
+
+;; Flatten nested route groups and instrument each route — the host/make-app seam.
+(define otel/instrument-routes
+  (fn (routes) (map otel/instrument-route (dr/flatten-routes routes))))