tls.x509_fp macro (#154)

Added tls.x509 key fingerprint macro.

Author: zsoltgyulai94

doc/_admin-guide/110_Template_and_rewrite/000_Customize_message_format/004_Macros_of_syslog-ng.md

@@ -503,6 +503,7 @@ following macros are available in {{ site.product.short_name }} version 3.9 and
 - .tls.x509_o: The value of the Organization field.
 
 - .tls.x509_ou: The value of the Organization Unit field.
+- .tls.x509_fp: The key fingerprint of the peer, when the trusted-key() option is used. Available in {{ site.product.short_name }} 4.8 and later versions.
 
 ## ${UNIQID}