add diff value of key-type key<new key> for each pattern matched.

[johnzheng1975]

In multi_format, add diff value of keytype key for each pattern matched.

<parse>
     @type multi_format
       <pattern>
           format /.*reg1.*/
          #keytype type1
       </pattern>
       <pattern>
           format /.*reg2.*/
          #keytype type2
       </pattern>
       <pattern>
           format json
           #keytype type3
       </pattern>
   </parse>

My purpose is use this for later filter/ or search/ ...

[johnzheng1975]

Or at least to support diff inject_key_prefix for diff pattern. Thanks.