out_es: change FLB_ES_DEFAULT_TAG_KEY to flb-key from _flb-key (#844)
out_es: change FLB_ES_DEFAULT_TAG_KEY to flb-key from _flb-key (#844)

Elasticsearch reserves keys starting with underscore (_) for internal
use. This is enforced in Kibana. Change the default to avoid errors.

Signed-off-by: Don Bowman <1f1e67e5fdb25d2e5cd18ddc0fee425272daab56@agilicus.com>
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