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/tailer.go

@@ -52,7 +52,7 @@ func (mt *MultiTailer) Run(ctx context.Context) {
 	if err != nil {
 		log.Fatalf("tailer: failed to create watcher: %v", err)
 	}
-	defer watcher.Close()
+	defer func() { _ = watcher.Close() }()
 
 	files := make(map[string]*fileState)
 	retrying := make(map[string]struct{}) // paths currently in a retryOpen goroutine
@@ -85,7 +85,7 @@ func (mt *MultiTailer) Run(ctx context.Context) {
 		select {
 		case <-ctx.Done():
 			for _, fs := range files {
-				fs.f.Close()
+				_ = fs.f.Close()
 			}
 			return
 
@@ -117,7 +117,7 @@ func (mt *MultiTailer) Run(ctx context.Context) {
 			if event.Has(fsnotify.Rename) || event.Has(fsnotify.Remove) {
 				// Drain remaining bytes in the old fd before it disappears.
 				mt.readLines(fs.reader)
-				fs.f.Close()
+				_ = fs.f.Close()
 				delete(files, event.Name)
 				_ = watcher.Remove(event.Name)
 				startRetry(event.Name)
@@ -167,7 +167,7 @@ func (mt *MultiTailer) rescan(
 	for path, fs := range files {
 		if _, matched := current[path]; !matched {
 			mt.readLines(fs.reader)
-			fs.f.Close()
+			_ = fs.f.Close()
 			_ = watcher.Remove(path)
 			delete(files, path)
 			log.Printf("tailer: retired %s (no longer matched by any pattern)", path)
@@ -182,11 +182,11 @@ func openAndSeekEOF(path string, watcher *fsnotify.Watcher) (*fileState, error)
 		return nil, err
 	}
 	if _, err := f.Seek(0, io.SeekEnd); err != nil {
-		f.Close()
+		_ = f.Close()
 		return nil, err
 	}
 	if err := watcher.Add(path); err != nil {
-		f.Close()
+		_ = f.Close()
 		return nil, err
 	}
 	return &fileState{f: f, reader: bufio.NewReader(f)}, nil