fix: add error message

Author: zlodes

src/KeySerialization/JsonSerializer.php

@@ -48,7 +48,7 @@ public function unserialize(string $key): MetricNameWithLabels
                 : [];
 
             Assert::isArray($labels);
-            Assert::allStringNotEmpty($labels);
+            Assert::allStringNotEmpty($labels, 'Labels keys and values must be non-empty strings');
             /** @psalm-var array<non-empty-string, non-empty-string> $labels */
 
             return new MetricNameWithLabels($name, $labels);

tests/KeySerialization/JsonSerializerTest.php

@@ -98,7 +98,7 @@ public static function unserializationErrorsDataProvider(): iterable
 
         yield 'empty label' => [
             'foo|{"bar":""}',
-            'Expected a different value than ""',
+            'Labels keys and values must be non-empty strings',
         ];
 
         yield 'malformed json' => [