"emit_unmatched_lines" ommits source_hostname_key or source_address_key

[aeber]

Describe the bug
#2499 introduced the option emit_unmatched_lines to the syslog input to keep messages in the pipeline even if parsing fails.
Due to the way this was implemented source_hostname_key or source_address_key do not get included in the emitted message even though this information is provided by the underlying udp/tcp server.

fluentd/lib/fluent/plugin/in_syslog.rb

Lines 208 to 210 in 9e88103

	if @emit_unmatched_lines
	emit("#{@tag}.unmatched", Fluent::EventTime.now, {"unmatched_line" => data})
	end

fluentd/lib/fluent/plugin/in_syslog.rb

Lines 220 to 222 in 9e88103

	if @emit_unmatched_lines
	emit("#{@tag}.unmatched", Fluent::EventTime.now, {"unmatched_line" => text})
	end

fluentd/lib/fluent/plugin/in_syslog.rb

Lines 240 to 242 in 9e88103

	if @emit_unmatched_lines
	emit("#{@tag}.unmatched", Fluent::EventTime.now, {"unmatched_line" => text})
	end

To Reproduce

• Create syslog input with emit_unmatched_lines true and source_hostname_key source_hostname and inject some unparseable message.
• the emitted message only contains the key unmatched_line and no source_hostname

Expected behavior
source_hostname_key and source_address_key should be honored even for unmatched lines.

Your Environment

• Fluentd or td-agent version: fluentd 1.6.3
• Operating system: Debian 9