Copy \MongoDB\generate_index_name into our code

This function has been relocated to a private method in mongodb/mongo-php-library=1.16.0.
See:
- mongodb/mongo-php-library@bf36331
- mongodb/mongo-php-library#1092

We're copying it inline in our code here for compatibility with 1.16.0.

Author: spantaleev

lib/Mongo/MongoCollection.php

@@ -571,6 +571,37 @@ public function findOne($query = [], array $fields = [], array $options = [])
         return $document;
     }
 
+    /**
+     * Generate an index name from a key specification.
+     *
+     * @internal
+     * @param array|object $document Document containing fields mapped to values,
+     *                               which denote order or an index type
+     * @throws \MongoDB\Exception\InvalidArgumentException
+     */
+    static private function generate_index_name($document): string
+    {
+        if ($document instanceof Serializable) {
+            $document = $document->bsonSerialize();
+        }
+
+        if (is_object($document)) {
+            $document = get_object_vars($document);
+        }
+
+        if (! is_array($document)) {
+            throw \MongoDB\Exception\InvalidArgumentException::invalidType('$document', $document, 'array or object');
+        }
+
+        $name = '';
+
+        foreach ($document as $field => $type) {
+            $name .= ($name != '' ? '_' : '') . $field . '_' . $type;
+        }
+
+        return $name;
+    }
+
     /**
      * Creates an index on the given field(s), or does nothing if the index already exists
      *
@@ -597,7 +628,7 @@ public function createIndex($keys, array $options = [])
         }
 
         if (! isset($options['name'])) {
-            $options['name'] = \MongoDB\generate_index_name($keys);
+            $options['name'] = self::generate_index_name($keys);
         }
 
         $indexes = iterator_to_array($this->collection->listIndexes());