Some metrics can be hidden during APC warmup

[b-viguier]

Hello 👋

I was testing this library (through Artprima's SF bundle) and I discovered that during a new k8s deployment, some metrics can disappear (or be back).

According to my investigations, this is due to a race condition in the addItemToKey function:

prometheus_client_php/src/Prometheus/Storage/APCng.php

Lines 267 to 279 in 3e4811f

	private function addItemToKey(string $key, string $item): void
	{
	// Modify serialized array stored in $key
	$arr = apcu_fetch($key);
	if (false === $arr) {
	$arr = [];
	}
	$_item = $this->encodeLabelKey($item);
	if (!array_key_exists($_item, $arr)) {
	$arr[$_item] = 1;
	apcu_store($key, $arr, 0);
	}
	}

It reads the (array) value stored in APC, modifies it and stores it back to APC. Unfortunately, during a new deployment (in k8s) with an empty APC, several items need to be stored in APC concurrently. Then, when trying to add some new label values from 2 distinct queries, there are some chances to override one of the 2 modifications.

I didn't find any easy workaround...

Anyway, thank you very much for your work on this library 🙇