### 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
with conf_vars(
            {
                ("metrics", "statsd_on"): "True",
                ("metrics", "statsd_custom_client_path"): f"{__name__}.CustomStatsd",
            }
        ):
            importlib.reload(airflow.stats)
            assert isinstance(airflow.stats.Stats.statsd, CustomStatsd)
        
        importlib.reload(airflow.stats)
    def test_load_invalid_custom_stats_client(self):
        with conf_vars(
            {
                ("metrics", "statsd_on"): "True",
                ("metrics", "statsd_custom_client_path"): f"{__name__}.InvalidCustomStatsd",
            }
        ), pytest.raises(
            AirflowConfigException,
            match=re.escape(
                'Your custom Statsd client must extend the statsd.'
                'StatsClient in order to ensure backwards compatibility.'
            ),
        ):
            importlib.reload(airflow.stats)
            airflow.stats.Stats.incr("dummy_key")
        importlib.reload(airflow.stats)
class TestDogStats(unittest.TestCase):