fix(prebid-analytics): sample once per auction, not twice

lib/addons/prebid/analytics.test.ts

@@ -692,7 +692,8 @@ describe("OptablePrebidAnalytics", () => {
       expect(mockOptableInstance.witness).not.toHaveBeenCalled();
     });
 
-    it("should not send when sampling returns false", async () => {
+    it("should not sample - trackAuctionEnd already decided", async () => {
+      // Sampling here as well would make the effective rate rate^2.
       analytics = new OptablePrebidAnalytics(mockOptableInstance, {
         analytics: true,
         samplingRate: 0,
@@ -701,11 +702,11 @@ describe("OptablePrebidAnalytics", () => {
       const result = await analytics.sendToWitnessAPI("test.event", { prop: "value" });
 
       expect(result).toEqual({
-        disabled: true,
+        disabled: false,
         eventName: "test.event",
         properties: { prop: "value" },
       });
-      expect(mockOptableInstance.witness).not.toHaveBeenCalled();
+      expect(mockOptableInstance.witness).toHaveBeenCalledWith("test.event", { prop: "value" });
     });
 
     it("should send to witness API when enabled and sampled", async () => {
@@ -719,6 +720,41 @@ describe("OptablePrebidAnalytics", () => {
       expect(mockOptableInstance.witness).toHaveBeenCalledWith("test.event", { prop: "value" });
     });
 
+    it("should draw once per auction at a fractional rate", async () => {
+      // Regression: the rate was applied at trackAuctionEnd and again on send,
+      // so 0.1 behaved as 0.01. The second draw here fails the rate - if it
+      // still gated, the auction would never reach witness.
+      const randomSpy = jest.spyOn(Math, "random").mockReturnValueOnce(0.3).mockReturnValue(0.9);
+      jest.useFakeTimers();
+
+      analytics = new OptablePrebidAnalytics(mockOptableInstance, {
+        analytics: true,
+        samplingRate: 0.5,
+      });
+
+      await analytics.trackAuctionEnd({
+        auctionId: "auction-fractional",
+        timeout: 3000,
+        bidderRequests: [
+          {
+            bidderCode: "bidder1",
+            bidderRequestId: "req-1",
+            ortb2: { site: { domain: "example.com" }, user: { eids: [] } },
+            bids: [],
+          },
+        ],
+        bidsReceived: [],
+        noBids: [],
+      });
+      await jest.runAllTimersAsync();
+
+      expect(mockOptableInstance.witness).toHaveBeenCalledTimes(1);
+      expect(randomSpy).toHaveBeenCalledTimes(1);
+
+      jest.useRealTimers();
+      randomSpy.mockRestore();
+    });
+
     it("should handle errors from witness API", async () => {
       const error = new Error("Witness API error");
       mockOptableInstance.witness = jest.fn().mockRejectedValue(error);

lib/addons/prebid/analytics.ts

@@ -161,7 +161,10 @@ class OptablePrebidAnalytics {
   }
 
   /**
-   * Send an event to the Witness API when analytics are enabled and sampling passes.
+   * Send an event to the Witness API when analytics are enabled.
+   *
+   * Does not sample: `trackAuctionEnd` already decided that once per auction.
+   * Sampling again here would make the effective rate rate^2.
    * @param eventName - The name of the event to send (e.g. "optable.prebid.auction").
    * @param properties - An object of event properties to include in the payload.
    * @returns A small result object indicating whether the call was disabled or sent.
@@ -172,11 +175,6 @@ class OptablePrebidAnalytics {
       return { disabled: true, eventName, properties };
     }
 
-    if (!this.shouldSample()) {
-      this.log("Event not sampled - skipping Witness API call for:", eventName, properties);
-      return { disabled: true, eventName, properties };
-    }
-
     try {
       await this.optableInstance.witness(eventName, properties);
       this.log("Sending to Witness API:", eventName, properties);