out_stackdriver: fix bug in k8s_pod process_local_resource_id

[ryanohnemus]

The k8s_pod process_local_resource_id does not process the local_resource_id if using a custom_k8s_regex.

The return that is being checked if it is 0, should actually match what is happening in k8s_container and k8s_node sections: https://github.com/fluent/fluent-bit/blob/master/plugins/out_stackdriver/stackdriver.c#L1947 because the return should be > 0 when there is an actual match.

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[braydonk]

I wish this code used named constants for success and failure instead of magic numbers 0, -1, etc. to make it so this sort of thing doesn't go unnoticed this long. Thank you for catching this!