match_regex: Accept all data types of meta data

match_regex: Accept all data types of meta data

match_regex currently accepts only string type of meta data, but
there is no reason to reject other data types since they can be
easily stringified by meta_data_as_string().
Accepting all data type will provide more benefit, for example
the following config can distinguish that data is local or remote,
the previous match_regex outputs many error logs for this config
though:

  <Match "regex">
    MetaData "network:received" "true"
  </Match>

Signed-off-by: Takuro Ashie <ashie@clear-code.com>