docs: adds documentation about the log notification

Changelog-None: docs: adds documentation about the log notification
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

doc/developers-guide/plugin-development/event-notifications.md

@@ -185,6 +185,8 @@ A notification for topic `warning` is sent every time a new `BROKEN`/`UNUSUAL` l
    `jcon fd <error_fd_to_jsonrpc>:`, `plugin-manager`;
 4. `log` is the context of the original log entry.
 
+There is also a more general version of this notification called `log`, which has the same payload. This needs to be used with caution, but it is useful for plugins that report logs remotely. For example: using OpenTelemetry.
+
 ### `forward_event`
 
 A notification for topic `forward_event` is sent every time the status of a forward payment is set. The json format is same as the API `listforwards`.

lightningd/notification.c

@@ -129,7 +129,7 @@ static void log_notification_serialize(struct json_stream *stream,
 	if (l->level >= LOG_UNUSUAL)
 		json_object_start(stream, "warning");
 	else
-		json_object_start(stream, NULL);
+		json_object_start(stream, "log");
 	if (l->level >= LOG_UNUSUAL)
 		/* Choose "BROKEN"/"UNUSUAL" to keep consistent with the habit
 		 * of plugin. But this may confuses the users who want to 'getlog'

lightningd/plugin.c

@@ -1,3 +1,4 @@
+#include "ccan/str/str.h"
 #include "config.h"
 #include <ccan/array_size/array_size.h>
 #include <ccan/ccan/tal/grab_file/grab_file.h>
@@ -2324,8 +2325,11 @@ static bool plugin_subscriptions_contains(struct plugin *plugin,
 {
 	for (size_t i = 0; i < tal_count(plugin->subscriptions); i++) {
 		if (streq(method, plugin->subscriptions[i])
-		    /* Asterisk is magic "all" */
-		    || streq(plugin->subscriptions[i], "*")) {
+		    /* Asterisk is magic "all", and log notification
+		     * is a special notification that must be turn on
+		     * only if requested. */
+		    || (streq(plugin->subscriptions[i], "*")
+			&& !streq(plugin->subscriptions[i], "log"))) {
 			return true;
 		}
 	}

plugins/libplugin.c

@@ -1607,7 +1607,11 @@ static void ld_command_handle(struct plugin *plugin,
 		for (size_t i = 0; i < plugin->num_notif_subs; i++) {
 			if (streq(cmd->methodname,
 				  plugin->notif_subs[i].name)
-			    || streq(plugin->notif_subs[i].name, "*")) {
+			    /* * is a special case that means all, but log is another special
+			     * case that can impact performance so this notification is turned
+			     * only when directly specified by a plugin. */
+			    || (streq(plugin->notif_subs[i].name, "*")
+				&& !streq(plugin->notif_subs[i].name, "log"))) {
 				plugin->notif_subs[i].handle(cmd,
 							     plugin->buffer,
 							     paramstok);