Adding "prefix" to log_file_prefix causes logstash to fail

[idsvandermolen]

When you add a prefix to log_file_prefix field, logstash fails with an error.
Config:

input {
  generator { }
}
output {
  google_cloud_storage {
    bucket => "my-logs"
    temp_directory => "/tmp/logstash-gcs"
    log_file_prefix => "/test/logstash_gcs"
    ...
  }
}

Error:

[2020-02-26T16:00:20,738][ERROR][logstash.javapipeline    ][main] Pipeline aborted due to error {:pipeline_id=>"main", :exception=>#<Errno::ENOENT: No such file or directory - /tmp/logstash-gcs/test/logstash_gcs_cw054563.local_2020-02-26T16:00.part001.log.gz>, :backtrace=>["org/jruby/RubyIO.java:1239:in `sysopen'"

Please align features with logstash-s3-output plugin.