filter_aws: retrieve metadata on initialization
filter_aws: retrieve metadata on initialization

Current implementation try to retrieve metadata in the flush
callback, it works, but ideally we should do this in the initialization
stage and only 're-check' in the flush callback if is set or not.

This patch makes the plugin to retrieve metadata when the plugin gets
initialized.

Signed-off-by: Eduardo Silva <81f705dc2ce1a61a2621e0e4b442a9474e1d0c70@calyptia.com>

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