Support dynamic target field names.

It's conceivable that one would want the target field name to be
chosen dynamically using the contents of another field, so pass the
target option value through sprintf.

Author: magnusbaeck

lib/logstash/filters/json.rb

@@ -83,7 +83,7 @@ def filter(event)
     end
 
     if @target
-      event.set(@target, parsed)
+      event.set(event.sprintf(@target), parsed)
     else
       unless parsed.is_a?(Hash)
         @tag_on_failure.each{|tag| event.tag(tag)}

spec/filters/json_spec.rb

@@ -41,6 +41,22 @@
     end
   end
 
+  describe "parse message into a dynamically named target field" do
+    config <<-CONFIG
+      filter {
+        json {
+          # Parse message as JSON, store the results in the 'data' field'
+          source => "message"
+          target => "%{target}"
+        }
+      }
+    CONFIG
+
+    sample({"target" => "data", "message" => '{ "hello": "world" }'}) do
+      insist { subject.get("data")["hello"] } == "world"
+    end
+  end
+
   describe "tag invalid json" do
     config <<-CONFIG
       filter {