Live: make maximum WebSocket message size configurable (#99770)

Co-authored-by: Chris Marchbanks <chris.marchbanks@grafana.com>

pkg/setting/setting.go

@@ -435,6 +435,9 @@ type Cfg struct {
 	// LiveAllowedOrigins is a set of origins accepted by Live. If not provided
 	// then Live uses AppURL as the only allowed origin.
 	LiveAllowedOrigins []string
+	// LiveMessageSizeLimit is the maximum size in bytes of Websocket messages
+	// from clients. Defaults to 64KB.
+	LiveMessageSizeLimit int
 
 	// Grafana.com URL, used for OAuth redirect.
 	GrafanaComURL string
@@ -1974,6 +1977,10 @@ func (cfg *Cfg) readLiveSettings(iniFile *ini.File) error {
 	if cfg.LiveMaxConnections < -1 {
 		return fmt.Errorf("unexpected value %d for [live] max_connections", cfg.LiveMaxConnections)
 	}
+	cfg.LiveMessageSizeLimit = section.Key("message_size_limit").MustInt(65536)
+	if cfg.LiveMessageSizeLimit < -1 {
+		return fmt.Errorf("unexpected value %d for [live] message_size_limit", cfg.LiveMaxConnections)
+	}
 	cfg.LiveHAEngine = section.Key("ha_engine").MustString("")
 	switch cfg.LiveHAEngine {
 	case "", "redis":