Send fx's own events through the service's logger (closes #183)

fx printed its dependency graph and its start and stop hooks through
its console logger on standard error. fx.WithLogger now hands those
events to the service's logger: building the graph at DEBUG, the hooks,
the start and the stopping signal at INFO, failures at ERROR. The fx
logger's constructor takes the configuration, so the level DEBUG=true
sets is in force before fx replays the events it held back; a failure
before that, such as an invalid configuration value, still goes to
fx's console logger. FxLogger in internal/logger holds one SlogLogger
per level. fx moves from v1.20.1, which has no SlogLogger, to v1.24.0.
The README names the Go runtime and that early failure as writers
outside internal/logger.

Model: opus-5-5

internal/logger/logger_test.go

@@ -1,13 +1,23 @@
 package logger_test
 
 import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"log/slog"
 	"testing"
 
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/fx"
+	"go.uber.org/fx/fxevent"
 	"go.uber.org/fx/fxtest"
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/logger"
 )
 
+var errStopHook = errors.New("stop hook failed on purpose")
+
 func testGlobals() *globals.Globals {
 	return &globals.Globals{
 		Appname: "test-app",
@@ -57,3 +67,47 @@ func TestEnableDebugLogging(t *testing.T) {
 	// Test debug logging
 	l.Get().Debug("debug message", "test", true)
 }
+
+// TestFxLogger_Levels starts and stops an fx app that reports its own
+// events through NewFxLogger, as cmd/webhooker does, and reads back
+// what reached the handler: the graph at DEBUG, the start at INFO and
+// a failed stop hook at ERROR, each as a structured record.
+func TestFxLogger_Levels(t *testing.T) {
+	t.Parallel()
+
+	var out bytes.Buffer
+
+	log := slog.New(slog.NewJSONHandler(
+		&out, &slog.HandlerOptions{Level: slog.LevelDebug},
+	))
+
+	app := fx.New(
+		fx.WithLogger(func() fxevent.Logger {
+			return logger.NewFxLogger(log)
+		}),
+		fx.Invoke(func(lc fx.Lifecycle) {
+			lc.Append(fx.StopHook(func() error { return errStopHook }))
+		}),
+	)
+
+	require.NoError(t, app.Start(t.Context()))
+	require.ErrorIs(t, app.Stop(t.Context()), errStopHook)
+
+	levels := map[string]string{}
+
+	decoder := json.NewDecoder(&out)
+	for decoder.More() {
+		var record struct {
+			Level string `json:"level"`
+			Msg   string `json:"msg"`
+		}
+
+		require.NoError(t, decoder.Decode(&record))
+
+		levels[record.Msg] = record.Level
+	}
+
+	assert.Equal(t, "DEBUG", levels["provided"])
+	assert.Equal(t, "INFO", levels["started"])
+	assert.Equal(t, "ERROR", levels["OnStop hook failed"])
+}