Add async event loop lag metric

[dapplion]

We need a metric to track how "busy" our node is.

A potential approach would be to track async event loop lag:

setInterval(async () => {
  const startTime = Date.now();
  await sleep(0);
  const asyncEventLoopLag = Date.now() - startTime;
}, 1000)

Originally posted by @dapplion in #2952 (comment)

[dapplion]

prom-client should already have a similar metric. Check with libuv internals for more detailed metrics