[ResponseOps][taskManager] add debug log message when task ends

[pmuellr]

While dealing with a support issue, we got some Kibana logs with debug set on for task manager logging. Once nice thing is that it logs a message when tasks start:

[DEBUG][plugins.taskManager] Running task reports:monitor "reports:monitor"

However, it does NOT log a message when the task ends. It would be nice to have this as well.

An additional goodie would be to add some additional tag values to these messages, so we can filter on them in Discover/elsewhere. Thinking:

• task:start on the start message
• task:end on the end message

We could also be more elaborate, and add the task type and perhaps task id as tags. With newer Discovers, you can do some "popup aggs" on the term frequency of these, and then expand on that with full on Lens viz, which proved to be useful for the tagged event log delay messages, which we recently added:

kibana/x-pack/plugins/task_manager/server/task_running/task_runner.ts

Lines 622 to 631 in f149a17

	const { eventLoopBlockMs = 0 } = taskTiming;
	const taskLabel = `${this.taskType} ${this.instance.task.id}`;
	if (eventLoopBlockMs > this.eventLoopDelayConfig.warn_threshold) {
	this.logger.warn(
	`event loop blocked for at least ${eventLoopBlockMs} ms while running task ${taskLabel}`,
	{
	tags: [this.taskType, taskLabel, 'event-loop-blocked'],
	}
	);
	}

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)