Add file rotation based on file age to file output plugin (#5547)

plugins/outputs/file/README.md

@@ -8,6 +8,9 @@ This plugin writes telegraf metrics to files
   ## Files to write to, "stdout" is a specially handled file.
   files = ["stdout", "/tmp/metrics.out"]
 
+  ## If this is defined, files will be rotated by the time.Duration specified
+  # rotate_max_age = "1m"
+
   ## Data format to output.
   ## Each data format has its own unique set of configuration options, read
   ## more about them here: