fix: UDP listener parses batched datagrams

nginx-ipng-stats-plugin's ipng_stats_logtail directive buffers many log
lines into a single UDP datagram (default buffer=64k flush=1s). The
listener was treating each datagram as exactly one log line, so any
datagram with N>1 lines failed the v1 field-count check and dropped
silently. In production this showed up as logtail_udp_packets_received_total
roughly 4x logtail_udp_loglines_success_total — matching typical
burst-coalesced 4-lines-per-batch ratios.

Fix: strip trailing CRLF, split the payload on '\n', parse each
non-empty line independently. Counter semantics now match the names:

  packets_received  — datagrams off the socket (one per recvfrom)
  loglines_success  — log lines parsed OK (may be many per datagram)
  loglines_consumed — log lines forwarded to the store (not dropped)

After the fix, loglines_success ≈ packets_received × avg_lines_per_batch.

Regression test TestUDPListenerBatchedDatagram sends one datagram with
three '\n'-separated v1 lines and asserts all three LogRecords arrive,
plus loglines_success >= 3 * packets_received.

Docs (user-guide.md, design.md) now explain the datagram-vs-line unit
distinction so operators don't misread the ratio.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/collector/udp.go

@@ -74,21 +74,31 @@ func (u *UDPListener) Run(ctx context.Context) {
 		if u.prom != nil {
 			u.prom.IncUDPPacket()
 		}
-		line := strings.TrimRight(string(buf[:n]), "\r\n")
-		rec, ok := ParseUDPLine(line, u.v4bits, u.v6bits)
-		if !ok {
-			continue
-		}
-		if u.prom != nil {
-			u.prom.IncUDPSuccess()
-		}
-		select {
-		case u.ch <- rec:
-			if u.prom != nil {
-				u.prom.IncUDPConsumed()
+		// nginx-ipng-stats-plugin batches log lines into a single UDP
+		// datagram (default buffer=64k flush=1s), so one packet may carry
+		// many lines. nginx's log_format always ends a rendered line with
+		// '\n'; split on that and process each line independently.
+		payload := strings.TrimRight(string(buf[:n]), "\r\n")
+		for _, line := range strings.Split(payload, "\n") {
+			line = strings.TrimSuffix(line, "\r")
+			if line == "" {
+				continue
+			}
+			rec, ok := ParseUDPLine(line, u.v4bits, u.v6bits)
+			if !ok {
+				continue
+			}
+			if u.prom != nil {
+				u.prom.IncUDPSuccess()
+			}
+			select {
+			case u.ch <- rec:
+				if u.prom != nil {
+					u.prom.IncUDPConsumed()
+				}
+			default:
+				// Channel full — drop rather than block the read loop.
 			}
-		default:
-			// Channel full — drop rather than block the read loop.
 		}
 	}
 }