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/frontend/handler.go

@@ -542,7 +542,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			fmt.Sprintf("cannot connect to %s: %v", params.Target, err)))
 		return
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 
 	ctx, cancel := context.WithTimeout(r.Context(), 5*time.Second)
 	defer cancel()
@@ -589,7 +589,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		}
 		resp, err := ltClient.ListTargets(ctx, &pb.ListTargetsRequest{})
 		if ltConn != nil {
-			ltConn.Close()
+			_ = ltConn.Close()
 		}
 		if err != nil {
 			ltCh <- nil
@@ -683,5 +683,5 @@ func writeRawJSON(w http.ResponseWriter, params QueryParams, resp *pb.TopNRespon
 		o.Entries[i] = entry{Label: e.Label, Count: e.Count}
 	}
 	w.Header().Set("Content-Type", "application/json")
-	json.NewEncoder(w).Encode(o)
+	_ = json.NewEncoder(w).Encode(o)
 }