[Minor] Metadata exporter: fix custom_select/custom_format/custom_push
[Minor] Metadata exporter: fix custom_select/custom_format/custom_push

 - Also support adding metadata to HTTP headers
 - Add score to general metadata

AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd