instruction: Write Go code to go by example: rate limiting
output: ```dockerfile
  $ go run rate-limiting.go
  request 1 2012-10-19 00:38:18.687438 +0000 UTC
  request 2 2012-10-19 00:38:18.887471 +0000 UTC
  request 3 2012-10-19 00:38:19.087238 +0000 UTC
  request 4 2012-10-19 00:38:19.287338 +0000 UTC
  request 5 2012-10-19 00:38:19.487331 +0000 UTC
  ```