upstream: a diagnostics episode follows health, and a peer fault carries its cause

src/web/metrics.zig

@@ -1520,18 +1520,16 @@ const AnsweringClient = struct {
         io: std.Io,
         query: []const u8,
         response_buf: []u8,
-        selected: *?[]const u8,
-    ) transport.ExchangeError![]u8 {
+    ) transport.LeafError!transport.Outcome {
         _ = io;
         _ = query;
         const self: *AnsweringClient = @ptrCast(@alignCast(ptr));
         _ = self.calls.fetchAdd(1, .acq_rel);
-        selected.* = "fake://leaf";
         @memcpy(response_buf[0..reply.len], reply);
-        return response_buf[0..reply.len];
+        return .{ .reply = response_buf[0..reply.len] };
     }
 
-    fn client(self: *AnsweringClient) transport.Client {
+    fn leaf(self: *AnsweringClient) transport.Leaf {
         return .{ .ptr = self, .exchangeFn = exchangeFn };
     }
 };
@@ -1546,10 +1544,10 @@ test "the queue families carry what a real pool recorded, through the real snaps
     defer threaded.deinit();
     const io = threaded.io();
 
-    var leaf: AnsweringClient = .{};
+    var answering: AnsweringClient = .{};
     var slots = [_]pool_mod.Slot{
-        .{ .client = leaf.client() },
-        .{ .client = leaf.client() },
+        .{ .client = answering.leaf() },
+        .{ .client = answering.leaf() },
     };
     var recoveries: std.atomic.Value(u64) = .init(0);
     var entries = [_]pool_mod.Entry{.{
@@ -1569,7 +1567,7 @@ test "the queue families carry what a real pool recorded, through the real snaps
     var buf: [512]u8 = undefined;
     var selected: ?[]const u8 = null;
     _ = try pool.exchange(io, "\x12\x34\x01\x00", &buf, &selected);
-    try testing.expectEqual(@as(u32, 1), leaf.calls.load(.acquire));
+    try testing.expectEqual(@as(u32, 1), answering.calls.load(.acquire));
 
     // The counters a burst would move, written straight into the entry: this
     // test is about what the snapshot path carries, not about reproducing a