measurement: record the start of adaptive probing as a rate change

backend/shared/MoShiro.Measurement/Blocks/Http.cs

@@ -143,6 +143,7 @@ public sealed class HttpDialer : BlockBase
         }
 
         double max = Math.Max(Spec.Int("rate_max_per_min"), Spec.Int("rate_min_per_min")) / 60.0;
+        ReportStart(max);
         return new AdaptiveRate(new AdaptiveRateOptions
         {
             MinHz = Spec.Int("rate_min_per_min") / 60.0,