prober: record DERP dropped packets as they occur

Record dropped packets as soon as they time out, rather than after tx
record queues spill over, this will more accurately capture small
amounts of packet loss in a timely fashion.

Updates tailscale/corp#24522

Signed-off-by: James Tucker <james@tailscale.com>

prober/derp.go

@@ -425,6 +425,24 @@ func runDerpProbeQueuingDelayContinously(ctx context.Context, from, to *tailcfg.
 	txRecords := make([]txRecord, 0, packetsPerSecond*int(packetTimeout.Seconds()))
 	var txRecordsMu sync.Mutex
 
+	// applyTimeouts walks over txRecords and expires any records that are older
+	// than packetTimeout, recording in metrics that they were removed.
+	applyTimeouts := func() {
+		txRecordsMu.Lock()
+		defer txRecordsMu.Unlock()
+
+		now := time.Now()
+		recs := txRecords[:0]
+		for _, r := range txRecords {
+			if now.Sub(r.at) > packetTimeout {
+				packetsDropped.Add(1)
+			} else {
+				recs = append(recs, r)
+			}
+		}
+		txRecords = recs
+	}
+
 	// Send the packets.
 	sendErrC := make(chan error, 1)
 	// TODO: construct a disco CallMeMaybe in the same fashion as magicsock, e.g. magic bytes, src pub, seal payload.
@@ -445,10 +463,12 @@ func runDerpProbeQueuingDelayContinously(ctx context.Context, from, to *tailcfg.
 			case <-ctx.Done():
 				return
 			case <-t.C:
+				applyTimeouts()
 				txRecordsMu.Lock()
 				if len(txRecords) == cap(txRecords) {
 					txRecords = slices.Delete(txRecords, 0, 1)
 					packetsDropped.Add(1)
+					log.Printf("unexpected: overflow in txRecords")
 				}
 				txRecords = append(txRecords, txRecord{time.Now(), seq})
 				txRecordsMu.Unlock()