More useful toString on UpdateSettingsTask

[DaveCTurner]

Today UpdateSettingsTask#toString is just the default which is not helpful when it appears in logs. This commit makes it include the affected indices and settings.

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[DaveCTurner]

For the record the relevant toString() call is the one here:

elasticsearch/server/src/main/java/org/elasticsearch/cluster/ClusterStateTaskExecutor.java

Lines 71 to 82 in 73863dd

	default String describeTasks(List<T> tasks) {
	final StringBuilder output = new StringBuilder();
	Strings.collectionToDelimitedStringWithLimit(
	(Iterable<String>) () -> tasks.stream().map(Object::toString).filter(s -> s.isEmpty() == false).iterator(),
	", ",
	"",
	"",
	1024,
	output
	);
	return output.toString();
	}

We only call this if we need the full string description of the tasks for some reason (typically trace logging), otherwise this code is not called.

[idegtiarenko]

Should it mention anywhere that this is UpdateSettingsClusterStateUpdateRequest?
I wonder if it might not be immediately obvious what produced the string in case we have many strings from distinct cluster state update tasks.

[DaveCTurner]

We also include the source of the tasks (which is entry.getKey() here:

elasticsearch/server/src/main/java/org/elasticsearch/cluster/service/BatchSummary.java

Line 34 in a733f42

return tasks.isEmpty() ? entry.getKey() : entry.getKey() + "[" + tasks + "]";

For the update-settings tasks this includes the string update-settings so I think there is no chance of confusion here.

[idegtiarenko]

Added a non-blocking suggestion, otherwise LGTM