Logging: structured slog messages throughout, and WithLogger options so callers choose the logger

Log messages are lower-case event names with their details as key/value
attributes ("error" for errors), in place of formatted strings.

Loggable package take a WithLogger option as part of their "New*" call,
defaulting to slog.Default(). Each tags its lines with its `component`.

Some debug loggers print directly rather than using slog when in
terminal logging mode.

internal/websocket/hub.go

@@ -27,13 +27,35 @@ type Hub struct {
 // max_message_size is not set.
 const defaultMaxMessageSize = 32 << 10
 
-// NewHub creates a hub.
-func NewHub(conf config.JSCServerWebSocket, log *slog.Logger) *Hub {
-	return &Hub{
+// Option configures a Hub.
+type Option func(*Hub)
+
+// WithLogger makes the hub log to logger instead of slog.Default().
+func WithLogger(logger *slog.Logger) Option {
+	return func(h *Hub) {
+		h.log = logger
+	}
+}
+
+// NewHub creates a hub. It logs to slog.Default() unless given WithLogger.
+func NewHub(conf config.JSCServerWebSocket, opts ...Option) *Hub {
+	hub := &Hub{
 		conf:    conf,
-		log:     log.With("component", "ws"),
 		clients: make(map[*client]struct{}),
 	}
+	for _, opt := range opts {
+		opt(hub)
+	}
+	return hub
+}
+
+// logger returns the logger the hub logs to, tagged with its component.
+func (h *Hub) logger() *slog.Logger {
+	log := h.log
+	if log == nil {
+		log = slog.Default()
+	}
+	return log.With("component", "ws")
 }
 
 // Push sends a message of type msgType to every client. The payload is
@@ -45,7 +67,7 @@ func (h *Hub) Push(msgType string, payload any) {
 	if payload != nil {
 		var err error
 		if rawPayload, err = json.Marshal(payload); err != nil {
-			h.log.Error("marshal push payload", "type", msgType, "error", err)
+			h.logger().Error("marshal push payload", "type", msgType, "error", err)
 			return
 		}
 	}
@@ -55,7 +77,7 @@ func (h *Hub) Push(msgType string, payload any) {
 		Payload: rawPayload,
 	})
 	if err != nil {
-		h.log.Error("marshal push", "type", msgType, "error", err)
+		h.logger().Error("marshal push", "type", msgType, "error", err)
 		return
 	}
 
@@ -69,7 +91,7 @@ func (h *Hub) Push(msgType string, payload any) {
 	h.m.RUnlock()
 
 	for _, client := range slow {
-		h.log.Warn("dropping slow client", "client", client.id, "ip", client.ip)
+		h.logger().Warn("dropping slow client", "client", client.id, "ip", client.ip)
 		go client.close(ws.StatusPolicyViolation, "slow consumer")
 	}
 }