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/client.go

@@ -106,7 +106,7 @@ func (c *client) run(ctx context.Context) {
 		// consumer), after a failed write or ping, or by the caller.
 		c.conn.CloseNow() //nolint:errcheck,gosec // finished with; it may already be closed
 	default:
-		c.hub.log.Debug("read error", "client", c.id, "error", err)
+		c.hub.logger().Debug("read error", "client", c.id, "error", err)
 		c.conn.CloseNow() //nolint:errcheck,gosec // the connection has failed anyway
 	}
 
@@ -178,7 +178,7 @@ func (c *client) writeLoop(ctx context.Context) {
 			err := c.conn.Write(writeCtx, websocket.MessageText, msg)
 			cancel()
 			if err != nil {
-				c.hub.log.Debug("write failed", "client", c.id, "error", err)
+				c.hub.logger().Debug("write failed", "client", c.id, "error", err)
 				c.cancel()
 				return
 			}
@@ -215,7 +215,7 @@ func (c *client) pingLoop(ctx context.Context) {
 			err := c.conn.Ping(pingCtx)
 			cancel()
 			if err != nil {
-				c.hub.log.Debug("ping failed", "client", c.id, "error", err)
+				c.hub.logger().Debug("ping failed", "client", c.id, "error", err)
 				c.conn.CloseNow() //nolint:errcheck,gosec // dropping the client anyway
 				c.cancel()
 				return
@@ -240,11 +240,11 @@ func (c *client) trySend(msg []byte) bool {
 func (c *client) sendEnvelope(env Envelope) {
 	encoded, err := json.Marshal(env)
 	if err != nil {
-		c.hub.log.Error("marshal", "client", c.id, "error", err)
+		c.hub.logger().Error("marshal", "client", c.id, "error", err)
 		return
 	}
 	if !c.trySend(encoded) {
-		c.hub.log.Warn("dropping slow client", "client", c.id, "ip", c.ip)
+		c.hub.logger().Warn("dropping slow client", "client", c.id, "ip", c.ip)
 		go c.close(websocket.StatusPolicyViolation, "slow consumer")
 	}
 }
@@ -273,14 +273,18 @@ func (c *client) close(code websocket.StatusCode, reason string) {
 // level, with its size; at most logMessageLimit bytes of it are logged. It
 // does nothing unless debug logging is enabled.
 func (c *client) logMessage(ctx context.Context, event string, data []byte) {
-	if !c.hub.log.Enabled(ctx, slog.LevelDebug) {
+	logger := c.hub.logger()
+	if !logger.Enabled(ctx, slog.LevelDebug) {
 		return
 	}
 	logged, truncated := data, false
 	if len(logged) > logMessageLimit {
 		logged, truncated = logged[:logMessageLimit], true
 	}
-	c.hub.log.LogAttrs(ctx, slog.LevelDebug, event,
+	logger.LogAttrs(
+		ctx,
+		slog.LevelDebug,
+		event,
 		slog.String("client", c.id),
 		slog.Int("bytes", len(data)),
 		slog.String("message", string(logged)),