What are the best metrics to decide when a webserver in node to send 503 server busy response?

[mai1x9]

Let's say a web server written in node using http library or using frameworks like express/ fastify/ Koa etc..
When server is Busy usually 503 is sent, on what metrics shall on decide to send 503.

Some of the options I could figure out is

• event loop latency / lag detection using libraries like
  • blocked https://www.npmjs.com/package/blocked
  • event loop stats https://www.npmjs.com/package/event-loop-stats
  • toobusy-js for express https://www.npmjs.com/package/toobusy-js (available for Koa too)
  • using perf hooks and calculating event loop srats
• Based on number of http requests queued.
• Based om number of max concurrent users.
• Based on server metrics like cpu%, mem%

Are there any other better options when should one send 503?. Regarding event loop delay, let's say event loop is really blocked, then no other network requests shall be processed till next io cycle in event loop, then how would some send 503 when no request hits server.