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_test.go

@@ -17,7 +17,7 @@ func TestUDPListenerRoundTrip(t *testing.T) {
 		t.Fatalf("listen probe: %v", err)
 	}
 	addr := pc.LocalAddr().String()
-	pc.Close() // release; listener will re-bind
+	_ = pc.Close() // release; listener will re-bind
 
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
@@ -31,15 +31,15 @@ func TestUDPListenerRoundTrip(t *testing.T) {
 	if err != nil {
 		t.Fatalf("dial: %v", err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 
 	// The listener is started asynchronously; retry for up to 1s.
 	good := "v1\twww.example.com\t1.2.3.4\tGET\t/\t200\t42\t0.010\t0\t12345\tdirect\t10.0.0.1\thttps"
 	bad := "not enough\tfields"
 	deadline := time.Now().Add(time.Second)
 	for time.Now().Before(deadline) {
-		conn.Write([]byte(good))
-		conn.Write([]byte(bad))
+		_, _ = conn.Write([]byte(good))
+		_, _ = conn.Write([]byte(bad))
 		select {
 		case rec := <-ch:
 			if rec.Website != "www.example.com" || rec.SourceTag != "direct" {
@@ -80,7 +80,7 @@ func TestUDPListenerBatchedDatagram(t *testing.T) {
 		t.Fatalf("listen probe: %v", err)
 	}
 	addr := pc.LocalAddr().(*net.UDPAddr)
-	pc.Close()
+	_ = pc.Close()
 
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
@@ -100,7 +100,7 @@ func TestUDPListenerBatchedDatagram(t *testing.T) {
 	if err != nil {
 		t.Fatalf("src listen: %v", err)
 	}
-	defer src.Close()
+	defer func() { _ = src.Close() }()
 
 	// Drive the listener with retries until all three records land.
 	got := make(map[string]bool)
@@ -148,7 +148,7 @@ func TestUDPListenerMultipleSources(t *testing.T) {
 		t.Fatalf("listen probe: %v", err)
 	}
 	addr := pc.LocalAddr().(*net.UDPAddr)
-	pc.Close()
+	_ = pc.Close()
 
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
@@ -166,7 +166,7 @@ func TestUDPListenerMultipleSources(t *testing.T) {
 		if err != nil {
 			t.Fatalf("%s listen: %v", tag, err)
 		}
-		defer src.Close()
+		defer func() { _ = src.Close() }()
 		deadline := time.Now().Add(time.Second)
 		for time.Now().Before(deadline) {
 			if _, err := src.WriteTo(good, addr); err != nil {