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.

Makefile

@@ -5,7 +5,7 @@ PROTO_FILE   := $(PROTO_DIR)/logtail.proto
 GEN_FILES    := proto/logtailpb/logtail.pb.go proto/logtailpb/logtail_grpc.pb.go
 
 NATIVE_ARCH  := $(shell go env GOARCH)
-VERSION      := 0.9.2
+VERSION      := 1.0.1
 COMMIT_HASH  := $(shell git rev-parse --short HEAD 2>/dev/null || echo unknown)
 DATE         := $(shell date -u +%Y-%m-%dT%H:%M:%SZ)
 LDFLAGS      := -s -w \
@@ -28,7 +28,7 @@ GO_VERSION ?= 1.24.6
 # accepts. Older releases can't parse recent Go syntax.
 GOLANGCI_LINT_VERSION ?= 1.64.0
 
-.PHONY: help all build build-amd64 build-arm64 test lint proto pkg-deb docker docker-push clean \
+.PHONY: help all build build-amd64 build-arm64 test lint fixstyle proto pkg-deb docker docker-push clean \
         install-deps install-deps-apt install-deps-go install-deps-go-tools
 
 # help is the default target. Keep the list aligned with the .PHONY block above.
@@ -40,6 +40,7 @@ help:
 	@echo "  build-arm64          build all four binaries for linux/arm64 into build/arm64/"
 	@echo "  test                 run 'go test ./...'"
 	@echo "  lint                 run 'golangci-lint run ./...'"
+	@echo "  fixstyle             rewrite all .go files in place with 'gofmt -w'"
 	@echo "  proto                regenerate proto/logtailpb/*.pb.go from proto/logtail.proto"
 	@echo "  pkg-deb              build amd64 + arm64 .deb packages (requires dpkg-deb)"
 	@echo "  docker               buildx --load two tags ($(IMAGE):v$(VERSION), $(IMAGE):latest) for the native arch"
@@ -74,6 +75,11 @@ test: $(GEN_FILES)
 lint:
 	golangci-lint run ./...
 
+# fixstyle rewrites all tracked .go files with gofmt. Generated proto files are
+# excluded — they are regenerated from .proto via `make proto`.
+fixstyle:
+	gofmt -w $(shell find . -name '*.go' -not -path './proto/logtailpb/*')
+
 proto: $(GEN_FILES)
 
 # protoc's go_package option places output at the go_package path (not source-relative).