metrics: peak gauges for in-flight queries, stream connections and the query log queue

src/server/tcp_server_integration_test.zig

@@ -329,6 +329,52 @@ test "a canceled serve does not wait for a live connection" {
     try client_result;
 }
 
+test "the connections peak is the most slots held at once" {
+    if (!build_options.integration) return error.SkipZigTest;
+
+    const gpa = testing.allocator;
+    var threaded: std.Io.Threaded = .init(gpa, .{});
+    defer threaded.deinit();
+    const io = threaded.io();
+
+    var fake: FakeUpstream = .{ .reply = response_bytes };
+    var h_owner: upstream_owner.Borrowed = .{};
+    var h = bareHandler(h_owner.client(fake.client()));
+
+    const listen_address: net.IpAddress = try .parse("127.0.0.1", 0);
+    var server = try tcp_server.TcpServer.listen(gpa, io, listen_address, &h, .{
+        .max_connections = 3,
+        .idle_timeout = .{ .raw = .fromSeconds(600), .clock = .awake },
+    });
+    const server_address = server.boundAddress();
+
+    var serving: std.Io.Group = .init;
+    try serving.concurrent(io, tcp_server.TcpServer.serve, .{ &server, io });
+
+    var answered: [2]std.Io.Event = @splat(.unset);
+    var client_results: [2]anyerror!void = @splat({});
+    var clients_group: std.Io.Group = .init;
+    for (&answered, &client_results) |*event, *result| {
+        try clients_group.concurrent(io, holdConnection, .{ io, server_address, event, result });
+    }
+    for (&answered) |*event| event.waitUncancelable(io);
+
+    // Both replies prove both slots are claimed, and the listener has room for
+    // a third that never arrives.
+    const held_open = server.connectionsPeak();
+
+    server.deinit(io);
+    serving.await(io) catch |err| switch (err) {
+        error.Canceled => unreachable,
+    };
+    clients_group.await(io) catch |err| switch (err) {
+        error.Canceled => unreachable,
+    };
+
+    for (client_results) |result| try result;
+    try testing.expectEqual(@as(u64, 2), held_open);
+}
+
 /// The first slot the server would hand out, read after `serve` has returned so
 /// nothing can be writing it.
 fn firstFreeSlot(server: *const tcp_server.TcpServer) ?usize {