### 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
def setUp(self):
        pytest.importorskip('datadog')
        from datadog import DogStatsd
        self.dogstatsd_client = Mock(speck=DogStatsd)
        self.dogstats = SafeDogStatsdLogger(self.dogstatsd_client, AllowListValidator("stats_one, stats_two"))
    def test_increment_counter_with_allowed_key(self):
        self.dogstats.incr('stats_one')
        self.dogstatsd_client.increment.assert_called_once_with(
            metric='stats_one', sample_rate=1, tags=[], value=1
        )
    def test_increment_counter_with_allowed_prefix(self):
        self.dogstats.incr('stats_two.bla')
        self.dogstatsd_client.increment.assert_called_once_with(
            metric='stats_two.bla', sample_rate=1, tags=[], value=1
        )
    def test_not_increment_counter_if_not_allowed(self):
        self.dogstats.incr('stats_three')
        self.dogstatsd_client.assert_not_called()
def always_invalid(stat_name):
    raise InvalidStatsNameException(f"Invalid name: {stat_name}")
def always_valid(stat_name):
    return stat_name
class TestCustomStatsName(unittest.TestCase):