Installing the Spanner package currently throws a fatal error when minimum-stability:stable is used in composer:

Fatal error: Trait 'Google\Cloud\Core\LongRunning\LROTrait' not found in vendor/google/cloud-spanner/SpannerClient.php on line 65

This is because version 1.3 of google/cloud-core is being used due to the "dev" dependency on symfony/lock (see here).

I requested a tagged version of symfony/lock in symfony/symfony#22766, but until then this requires two fixes:

1. Require google/cloud-core:^1.4 in the spanner package
2. Move symfony/lock in google/cloud-core from "require" to "suggest".