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/cli/cli_test.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"bytes"
 	"context"
 	"encoding/json"
 	"net"
@@ -11,7 +10,6 @@ import (
 
 	pb "git.ipng.ch/ipng/nginx-logtail/proto/logtailpb"
 	"google.golang.org/grpc"
-	"google.golang.org/grpc/credentials/insecure"
 )
 
 // --- Unit tests ---
@@ -149,21 +147,11 @@ func startFake(t *testing.T, fs *fakeServer) string {
 	}
 	srv := grpc.NewServer()
 	pb.RegisterLogtailServiceServer(srv, fs)
-	go srv.Serve(lis)
+	go func() { _ = srv.Serve(lis) }()
 	t.Cleanup(srv.GracefulStop)
 	return lis.Addr().String()
 }
 
-func dialTest(t *testing.T, addr string) pb.LogtailServiceClient {
-	t.Helper()
-	conn, err := grpc.NewClient(addr, grpc.WithTransportCredentials(insecure.NewCredentials()))
-	if err != nil {
-		t.Fatal(err)
-	}
-	t.Cleanup(func() { conn.Close() })
-	return pb.NewLogtailServiceClient(conn)
-}
-
 // --- TopN tests ---
 
 func TestTopNSingleTarget(t *testing.T) {
@@ -225,23 +213,7 @@ func TestTopNJSON(t *testing.T) {
 	})
 
 	results := fanOutTopN([]string{addr}, nil, pb.GroupBy_WEBSITE, 10, pb.Window_W5M)
-	var buf bytes.Buffer
-	// Redirect stdout not needed; call JSON formatter directly.
 	r := results[0]
-	// Build expected JSON by calling printTopNJSON with a captured stdout.
-	// We test indirectly: marshal manually and compare fields.
-	type entry struct {
-		Label string `json:"label"`
-		Count int64  `json:"count"`
-	}
-	type out struct {
-		Source  string  `json:"source"`
-		Target  string  `json:"target"`
-		Entries []entry `json:"entries"`
-	}
-	_ = buf
-	_ = r
-	// Verify the response fields are correct for JSON serialization.
 	if r.resp.Source != "agg" {
 		t.Errorf("source = %q", r.resp.Source)
 	}