feat: log warning if log_key does not exist in log (#86)

Signed-off-by: Dominik Rosiek <drosiek@sumologic.com>

Author: sumo-drosiek

lib/fluent/plugin/out_sumologic.rb

@@ -367,6 +367,9 @@ def write(chunk)
       when 'logs'
         case log_format
         when 'text'
+          if !record.has_key?(@log_key)
+            log.warn "log key `#{@log_key}` has not been found in the log"
+          end
           log = log_to_str(record[@log_key])
         when 'json_merge'
           if @add_timestamp