add an example config for v0.12's filter

to be the basis for the upcoming documentation

example/v0_12_filter.conf

@@ -0,0 +1,78 @@
+# An example config to use filter plugins.
+# THIS FEATURE IS SUPPORTED FOR v0.12 AND ABOVE.
+
+# in_forward to generate events to be tested.
+# You can send an arbitrary event with an arbitrary tag.
+# For example, the following command creates the event
+# {"message":"hello world"} with tag = foo
+#
+# $ echo '{"message":"hello world"}' | fluent-cat foo
+
+<source>
+  type forward
+  port 24224
+</source>
+
+# For all events with the tag "foo", filter it out
+# UNLESS the value of the "message" field matches /keep this/
+#
+# - {"message":"keep this please"} is kept.
+# - {"message":"Do not keep"} is filtered out.
+# - {"messag22":"keep this please"} is filtered out.
+
+<filter foo>
+  type grep
+  regexp1  message keep this
+</filter>
+
+# Matches the events that was kept by the above filter
+<match foo>
+  type stdout
+</match>
+
+# For all events with the tag "bar", add the machine's hostname with
+# the key "hostname" BEFORE forwarding to another instance of Fluentd
+# at 123.4.2.4:24224.
+
+<filter bar>
+  type record_transformer
+  <record>
+    hostname ${hostname}
+  </record>
+</filter>
+
+# By the time it is getting matched here, the event has
+# the "hostname" field.
+<match bar>
+  type forward
+  <server>
+    host 123.4.2.4
+    port 24225
+  </server>
+</match>
+
+# Composing two filters. For all events with the tag foo.bar,
+# 
+# 1. The first filter filters out all events that has the field "hello"
+# 2. Then, for those events, we downcase the value of the "name" field.
+# 
+# - {"name":"SADA", "hello":100} gets filtered out
+# - {"name":"SADA"} becomes {"name":"sada"}
+# - {"last_name":"FURUHASHI"} throws an error because it has no field called "name"
+
+<filter foo.bar>
+  type grep
+  exclude1 hello .
+</filter>
+
+<filter foo.bar>
+  type record_transformer
+  enable_ruby true
+  <record>
+    name ${name.downcase}
+  </record>
+</filter>
+
+<match foo.bar>
+  type stdout
+</match>