### 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
pass
class InvalidCustomStatsd:
    """
    This custom Statsd class is invalid because it does not subclass
    statsd.StatsClient.
    """
    def __init__(self, host=None, port=None, prefix=None):
        pass
class TestStats(unittest.TestCase):
    def setUp(self):
        self.statsd_client = Mock(spec=statsd.StatsClient)
        self.stats = SafeStatsdLogger(self.statsd_client)
    def test_increment_counter_with_valid_name(self):
        self.stats.incr('test_stats_run')
        self.statsd_client.incr.assert_called_once_with('test_stats_run', 1, 1)
    def test_stat_name_must_be_a_string(self):
        self.stats.incr([])
        self.statsd_client.assert_not_called()
    def test_stat_name_must_not_exceed_max_length(self):
        self.stats.incr('X' * 300)
        self.statsd_client.assert_not_called()
    def test_stat_name_must_only_include_allowed_characters(self):