Bump cloud.google.com/go/spanner from 1.15.0 to 1.16.0

Bumps [cloud.google.com/go/spanner](https://github.com/googleapis/google-cloud-go) from 1.15.0 to 1.16.0.
- [Release notes](https://github.com/googleapis/google-cloud-go/releases)
- [Changelog](https://github.com/googleapis/google-cloud-go/blob/master/CHANGES.md)
- [Commits](googleapis/google-cloud-go@spanner/v1.15.0...spanner/v1.16.0)

Signed-off-by: dependabot[bot] <support@github.com>

go.mod

@@ -3,7 +3,7 @@ module github.com/sj14/dbbench
 go 1.14
 
 require (
-	cloud.google.com/go/spanner v1.15.0
+	cloud.google.com/go/spanner v1.16.0
 	github.com/denisenkom/go-mssqldb v0.0.0-20200910202707-1e08a3fab204
 	github.com/go-sql-driver/mysql v1.5.0
 	github.com/gocql/gocql v0.0.0-20200926162733-393f0c961220