milestone 31: concurrent upstream exchanges, dot session reuse, queue metrics

src/web/web_integration_test.zig

@@ -313,6 +313,8 @@ const Env = struct {
     pauser: pause_mod.Pause,
     tables: local_tables_mod.LocalTables,
     pool_entries: [1]pool_mod.Entry,
+    pool_slots: [1]pool_mod.Slot,
+    pool_recoveries: std.atomic.Value(u64),
     pool: pool_mod.Pool,
     state: server.WebState,
     web: server.Server,
@@ -387,14 +389,18 @@ const Env = struct {
         self.pauser = .{};
         self.tables = .empty;
 
+        // Never exchanged with: the pool feeds `/metrics` and the
+        // `/api/health` upstream condition only.
+        self.pool_slots = .{.{ .client = .{ .ptr = undefined, .exchangeFn = undefined } }};
+        self.pool_recoveries = .init(0);
         self.pool_entries = .{.{
             .endpoint = transport.Endpoint.parse("https://dns.example/dns-query") catch unreachable,
-            // Never exchanged with: the pool feeds `/metrics` and the
-            // `/api/health` upstream condition only.
-            .client = .{ .ptr = undefined, .exchangeFn = undefined },
+            .slots = &self.pool_slots,
             .priority = 1,
             .enabled = true,
             .health = .init,
+            .sem = .{ .permits = self.pool_slots.len },
+            .reuse_recoveries = &self.pool_recoveries,
         }};
         self.pool = .init(&self.pool_entries, .{}, .{
             .attempt = .{ .raw = .fromMilliseconds(50), .clock = .awake },