querylog: the writer queue defaults to 1024 entries, and a live stream allocates its ring on subscribe instead of at boot

src/web/metrics.zig

@@ -189,6 +189,9 @@ pub const Sample = struct {
     /// Exchanges the pool ended on this process's own resources. Pool-wide for
     /// the same reason as `upstream_budget_exhausted_total`.
     upstream_local_resource_total: ?u64 = null,
+    /// Live streams refused because their ring could not be allocated. Absent
+    /// while no hub is wired.
+    live_stream_allocation_failures_total: ?u64 = null,
     peaks: PeakSample = .{},
 };
 
@@ -307,6 +310,8 @@ pub fn collect(state: *server.WebState, io: std.Io, arena: Allocator) Allocator.
         }
     }
 
+    if (state.hub) |hub| sample.live_stream_allocation_failures_total = hub.ringAllocationFailures(io);
+
     return sample;
 }
 
@@ -525,6 +530,14 @@ pub fn render(w: *std.Io.Writer, sample: Sample) std.Io.Writer.Error!void {
         );
         try w.print("nxdns_upstream_local_resource_total {d}\n", .{total});
     }
+    if (sample.live_stream_allocation_failures_total) |total| {
+        try counter(
+            w,
+            "nxdns_live_stream_allocation_failures_total",
+            "Live query streams refused because their buffer could not be allocated.",
+            total,
+        );
+    }
 }
 
 fn renderPeaks(w: *std.Io.Writer, peaks: PeakSample) std.Io.Writer.Error!void {
@@ -1818,3 +1831,14 @@ test "the local-resource counter renders pool-wide, without a url label" {
     defer testing.allocator.free(absent);
     try testing.expect(!std.mem.containsAtLeast(u8, absent, 1, "nxdns_upstream_local_resource_total"));
 }
+
+test "live stream allocation failures render as a counter, absent without a hub" {
+    const text = try renderToString(testing.allocator, .{ .live_stream_allocation_failures_total = 2 });
+    defer testing.allocator.free(text);
+    try testing.expect(std.mem.containsAtLeast(u8, text, 1, "# TYPE nxdns_live_stream_allocation_failures_total counter\n"));
+    try testing.expect(std.mem.containsAtLeast(u8, text, 1, "nxdns_live_stream_allocation_failures_total 2\n"));
+
+    const absent = try renderToString(testing.allocator, .{});
+    defer testing.allocator.free(absent);
+    try testing.expect(!std.mem.containsAtLeast(u8, absent, 1, "nxdns_live_stream_allocation_failures"));
+}