The plugin can be configured as usual in the gerrit.config. All configuration is optional, if no values are provided these will be the defaults.
[plugin "spanner-refdb"]
gcpProject = test-project
spannerInstance = test-instance
spannerDatabase = global-refdb
plugin.spanner-refdb.gcpProject : Optional. The name of the google cloud platform project.
plugin.spanner-refdb.spannerInstance : Optional. The name of the spanner instance.
plugin.spanner-refdb.spannerDatabase : Optional. The name of the spanner database.