Add nested items support

lib/logstash/outputs/clickhouse.rb

@@ -131,16 +131,23 @@ def mutate( src )
     @mutations.each_pair do |dstkey, source|
       case source
         when String then
-          scrkey = source
-          next unless src.key?(scrkey)
-
-          res[dstkey] = src[scrkey]
+          if source.include?('][')
+            srcval = src.dig(*(source[1..-2].split('][')))
+          else
+            srcval = src.dig(source)
+          end
+          next if srcval.nil?
+          res[dstkey] = srcval
         when Array then
-          scrkey = source[0]
-          next unless src.key?(scrkey)
+          if source[0].include?('][')
+            srcval = src.dig(*(source[0][1..-2].split('][')))
+          else
+            srcval = src.dig(source[0])
+          end
+          next if srcval.nil?
           pattern = source[1]
           replace = source[2]
-          res[dstkey] = src[scrkey].sub( Regexp.new(pattern), replace )
+          res[dstkey] = srcval.sub( Regexp.new(pattern), replace )
       end
     end
     res