behavior about frequency

[islue]

my config:

{
    "servers": {
      "udp": {
        "port": 8125
      },
      "tcp": {
        "port": 8125
      }
    },
    "frequency": 10,
    "log_level": "debug",
    "backends": {
      "stdout": true
    }
}

environment: in a docker container with os amzn2

How to repeat:

1. start statsdcc with the config above
2. echo "foo:1|c" | nc -u 127.0.0.1 8125
3. wait time over 10s
4. echo "bar:1|c" | nc -u 127.0.0.1 8125
5. info about foo is printed out
6. bar won't be printed out if no more metric sent to statsdcc

I expected that statsdcc will flush metrics every 10s by automatically. But it seems that the flushing action must be triggered by a metric sending after 10s.
Is it a behavior by design? Or am I missing anything?