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/smoke_test.go

@@ -32,9 +32,18 @@ func BenchmarkIngest(b *testing.B) {
 	}
 }
 
-// BenchmarkParseLine measures parser throughput.
+// BenchmarkParseLine measures parser throughput on a v1 line.
 func BenchmarkParseLine(b *testing.B) {
-	line := "www.example.com\t1.2.3.4\t1741954800.123\tGET\t/api/v1/search?q=foo\t200\t1452\t0.043"
+	line := "v1\twww.example.com\t1.2.3.4\tGET\t/api/v1/search?q=foo\t200\t1452\t0.043\t0\t12345\tcdn\t10.0.0.1\thttps"
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		ParseLine(line, 24, 48)
+	}
+}
+
+// BenchmarkParseLineV2 measures parser throughput on a v2 line with upstream.
+func BenchmarkParseLineV2(b *testing.B) {
+	line := "v2\twww.example.com\t1.2.3.4\tGET\t/api/v1/search?q=foo\t200\t1500\t421\t0.043\t0.012\t200\t0\t12345\tcdn\t10.0.0.1\thttps"
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		ParseLine(line, 24, 48)
@@ -118,7 +127,7 @@ func TestGRPCEndToEnd(t *testing.T) {
 	}
 	grpcSrv := grpc.NewServer()
 	pb.RegisterLogtailServiceServer(grpcSrv, NewServer(store, "e2e-test"))
-	go grpcSrv.Serve(lis)
+	go func() { _ = grpcSrv.Serve(lis) }()
 	defer grpcSrv.Stop()
 
 	// Dial it
@@ -127,7 +136,7 @@ func TestGRPCEndToEnd(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 	client := pb.NewLogtailServiceClient(conn)
 
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)