### Apache Airflow version

2.1.2

### Operating System

Gentoo/Linux

### Versions of Apache Airflow Providers

_No response_

### Deployment

Docker-Compose

### Deployment details

_No response_

### What happened

In datadog-agent logs, got parsing error

[ AGENT ] 2021-09-29 03:20:01 UTC | CORE | ERROR | (pkg/dogstatsd/server.go:411 in errLog) | Dogstatsd: error parsing metric message '"airflow.dagrun.schedule_delay.skew:<Period [2021-09-29T03:20:00+00:00 -> 2021-09-29T03:20:00.968404+00:00]>|ms"': could not parse dogstatsd metric value: strconv.ParseFloat: parsing "<Period [2021-09-29T03:20:00+00:00 -> 2021-09-29T03:20:00.968404+00:00]>": invalid syntax

### What you expected to happen

since datadog agent expects a float, see https://github.com/DataDog/datadog-agent/blob/6830beaeb182faadac40368d9d781b796b4b2c6f/pkg/dogstatsd/parse.go#L119, the schedule_delay should be a float instead of timedelta.

### How to reproduce

_No response_

### Anything else

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

tests/core/test_stats.py
importlib.reload(airflow.stats)
        airflow.stats.Stats.incr("dummy_key")
        mock_statsd.return_value.incr.assert_called_once_with('dummy_key', 1, 1)
    @conf_vars(
        {
            ('metrics', 'statsd_datadog_enabled'): 'True',
            ('metrics', 'stat_name_handler'): 'tests.core.test_stats.always_valid',
        }
    )
    @mock.patch("datadog.DogStatsd")
    def test_does_send_stats_using_dogstatsd_when_the_name_is_valid(self, mock_dogstatsd):
        importlib.reload(airflow.stats)
        airflow.stats.Stats.incr("dummy_key")
        mock_dogstatsd.return_value.increment.assert_called_once_with(
            metric='dummy_key', sample_rate=1, tags=[], value=1
        )
    def tearDown(self) -> None:
        
        importlib.reload(airflow.stats)