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. fx's own SlogLogger takes one level for everything that is not a failure, so the small FxLogger in internal/logger holds one at each level and picks by event. fx moves from v1.20.1, which has no SlogLogger, to v1.24.0, its current release. The README's logging passage now names only the Go runtime, whose output cannot be redirected.

Model: opus-5-5

internal/logger/logger.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"go.uber.org/fx"
+	"go.uber.org/fx/fxevent"
 	"sneak.berlin/go/webhooker/internal/globals"
 )
 
@@ -106,3 +107,40 @@ func (l *Logger) Identify() {
 func (l *Logger) Writer() io.Writer {
 	return os.Stdout
 }
+
+// FxLogger writes fx's own events through a slog logger: how the
+// dependency graph was built at DEBUG, since it repeats on every
+// start; the start and stop hooks, the start itself and the signal
+// that stops the service at INFO; every failure at ERROR.
+//
+// The formatting is fx's own fxevent.SlogLogger. That logger takes a
+// single level for every event that is not a failure, so FxLogger
+// holds one at each level and picks between them.
+type FxLogger struct {
+	graph     *fxevent.SlogLogger
+	lifecycle *fxevent.SlogLogger
+}
+
+// NewFxLogger returns an FxLogger that writes through log.
+func NewFxLogger(log *slog.Logger) *FxLogger {
+	graph := &fxevent.SlogLogger{Logger: log}
+	graph.UseLogLevel(slog.LevelDebug)
+
+	lifecycle := &fxevent.SlogLogger{Logger: log}
+	lifecycle.UseLogLevel(slog.LevelInfo)
+
+	return &FxLogger{graph: graph, lifecycle: lifecycle}
+}
+
+// LogEvent implements fxevent.Logger.
+func (f *FxLogger) LogEvent(event fxevent.Event) {
+	switch event.(type) {
+	case *fxevent.Supplied, *fxevent.Provided, *fxevent.Replaced,
+		*fxevent.Decorated, *fxevent.BeforeRun, *fxevent.Run,
+		*fxevent.Invoking, *fxevent.Invoked,
+		*fxevent.LoggerInitialized:
+		f.graph.LogEvent(event)
+	default:
+		f.lifecycle.LogEvent(event)
+	}
+}