Added Gelfify filter

Added GELF output to support multiple (frist-value-match) level and facility sources
Added syslog support to named labels
(cherry picked from commit 8af7a1d)

lib/logstash/filters/gelfify.rb

@@ -0,0 +1,50 @@
+require "logstash/filters/base"
+require "logstash/namespace"
+
+# The GELFify filter parses RFC3164 severity levels to
+# corresponding GELF levels.
+class LogStash::Filters::Gelfify < LogStash::Filters::Base
+
+  config_name "gelfify"
+
+  public
+  def register
+
+    @syslog_level_map  = {
+        0 => 3, # Emergency => FATAL
+        1 => 5, # Alert     => WARN
+        2 => 3, # Critical  => FATAL
+        3 => 4, # Error     => ERROR
+        4 => 5, # Warning   => WARN
+        5 => 6, # Notice    => INFO
+        6 => 6, # Informat. => INFO
+        7 => 7  # Debug     => DEBUG
+    }
+
+    @logger.debug "Adding GELFify filter to type #{@type}"
+
+  end # def register
+
+  public
+  def filter(event)
+
+    return unless event.type == @type
+    @logger.debug "GELFIFY FILTER: received event of type #{event.type}"
+
+    if event.fields.include?("severity")
+
+        if @syslog_level_map[event.fields["severity"].to_i]
+            @logger.debug "GELFIFY FILTER: Severity level successfully mapped"
+            event.fields["GELF_severity"] = @syslog_level_map[event.fields["severity"].to_i]
+        end
+
+    else
+        @logger.warn "GELFIFY FILTER: No 'severity' field found"
+    end
+
+    if !event.cancelled?
+      filter_matched(event)
+    end
+  end # def filter
+
+end # class LogStash::Filters::Gelfify