Ensure that label values are strings in APCNg adapter (#159)

* Ensure that label values are strings in APCNg adapter

APCNg is crashing if the label value provided wasn't a string, but something that can be coerced to string (such as int).

The problem occurs in two different places:
- when a metric is emitted, the `storeLabelKeys()` calls into `addItemToKey()` which has its second parameter type hinted as a `string` and throws a type error if anything else is passed. This results in partially stored state;
- when trying to scrape metrics with partially stored state, the `APCng::collect()` will try to build all the permutations and expect all the key-value pairs for labels to exist, but numeric label values aren't persisted and so it will cause the `Undefined array key` error as reported in #154;

This change ensures that label values are cast to the string type before encoding them and using as APC keys.

Signed-off-by: Garry Filakhtov <garry.filakhtov@bigcommerce.com>

* Fix PHPStan issues
Signed-off-by: Garry Filakhtov <garry.filakhtov@bigcommerce.com>

---------

Signed-off-by: Garry Filakhtov <garry.filakhtov@bigcommerce.com>

Author: software-artificer

src/Prometheus/Storage/APCng.php

@@ -254,7 +254,7 @@ private function storeLabelKeys(array $data): void
                 $data['name'],
                 $label,
                 'label'
-            ]), isset($data['labelValues']) ? $data['labelValues'][$seq] : ''); // may not need the isset check
+            ]), isset($data['labelValues']) ? (string)$data['labelValues'][$seq] : ''); // may not need the isset check
         }
     }
 
@@ -860,7 +860,7 @@ private function sortSamples(array &$samples): void
      */
     private function encodeLabelValues(array $values): string
     {
-        $json = json_encode($values);
+        $json = json_encode(array_map("strval", $values));
         if (false === $json) {
             throw new RuntimeException(json_last_error_msg());
         }

tests/Test/Prometheus/Storage/APCngTest.php

@@ -42,6 +42,26 @@ public function itShouldNotClearWholeAPCacheOnFlush(): void
         );
     }
 
+    /**
+     * @test
+     */
+    public function nonStringLabelValuesAreCastToStrings(): void
+    {
+        apcu_clear_cache();
+
+        $adapter = new APCng();
+        $registry = new CollectorRegistry($adapter);
+        $registry->getOrRegisterCounter(
+            'ns',
+            'int_label_values',
+            'test int label values',
+            ['int_label'],
+        )->incBy(3, [3]);
+
+        $counter = apcu_fetch("prom:counter:ns_int_label_values:WyIzIl0=:value");
+        self::assertSame(3000, $counter);
+    }
+
     /**
      * @test
      */