chore(spanner): pin stats package to latest (GoogleCloudPlatform#1459)

Author: larkee

spanner/src/create_client_with_query_options.php

@@ -42,11 +42,11 @@ function create_client_with_query_options($instanceId, $databaseId)
     $spanner = new SpannerClient([
         'queryOptions' => [
             'optimizerVersion' => '1',
-            // Pin the statistics package used for this client instance to an
-            // older version. The list of available statistics packages can be
+            // Pin the statistics package used for this client instance to the
+            // latest version. The list of available statistics packages can be
             // found by querying the "INFORMATION_SCHEMA.SPANNER_STATISTICS"
             // table.
-            'optimizerStatisticsPackage' => 'auto_20191128_14_47_22UTC'
+            'optimizerStatisticsPackage' => 'latest'
         ]
     ]);
     $instance = $spanner->instance($instanceId);