RELEASE 1.0.1: v2 log format, source_tag-labeled metrics, lint cleanup

Wire-format and metric overhaul. Both file and UDP ingest now share one
versioned ParseLine that dispatches on the v<N>\t prefix; v1 stays
unchanged, v2 adds $bytes_sent (replacing $body_bytes_sent),
$request_length, $upstream_response_time, and $upstream_status. File
ingest gains the same versioning, and the legacy positional file format
is removed (no live deployments).

Prometheus exposition is rewritten:

  - nginx_http_bytes_sent and nginx_http_request_duration_seconds gain
    a source_tag label.
  - nginx_http_requests_by_source_total gains status_class.
  - New v2-only metrics: nginx_http_request_bytes,
    nginx_http_upstream_duration_seconds,
    nginx_http_upstream_requests_total{status_class}.
  - Dropped nginx_http_response_body_bytes_by_source (subsumed by the
    dual-labeled bytes_sent metric).

Adds 'make fixstyle' (gofmt -w) and clears all golangci-lint findings
across the repo (errcheck, S1001, ST1005, unused).

Docs in design.md FR-2/FR-8 and user-guide.md are rewritten to present
v2 as the recommended log format.

cmd/collector/udp.go

@@ -16,7 +16,7 @@ const udpReadBufBytes = 4 << 20
 const udpPacketBuf = 64 << 10
 
 // UDPListener receives nginx_ipng_stats_logtail datagrams on a local socket,
-// parses each packet as one log line, and forwards LogRecords to ch.
+// parses each line through the versioned ParseLine, and forwards LogRecords to ch.
 type UDPListener struct {
 	addr   string
 	v4bits int
@@ -50,7 +50,7 @@ func (u *UDPListener) Run(ctx context.Context) {
 	if err != nil {
 		log.Fatalf("udp: listen %s: %v", u.addr, err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 	if err := conn.SetReadBuffer(udpReadBufBytes); err != nil {
 		log.Printf("udp: SetReadBuffer(%d): %v", udpReadBufBytes, err)
 	}
@@ -58,7 +58,7 @@ func (u *UDPListener) Run(ctx context.Context) {
 
 	go func() {
 		<-ctx.Done()
-		conn.Close()
+		_ = conn.Close()
 	}()
 
 	buf := make([]byte, udpPacketBuf)
@@ -84,7 +84,7 @@ func (u *UDPListener) Run(ctx context.Context) {
 			if line == "" {
 				continue
 			}
-			rec, ok := ParseUDPLine(line, u.v4bits, u.v6bits)
+			rec, ok := ParseLine(line, u.v4bits, u.v6bits)
 			if !ok {
 				continue
 			}