### 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)

airflow/stats.py
"""Check if stat name contains invalid characters.
    Log and not emit stats if name is invalid
    """
    @wraps(fn)
    def wrapper(_self, stat=None, *args, **kwargs):
        try:
            if stat is not None:
                handler_stat_name_func = get_current_handler_stat_name_func()
                stat = handler_stat_name_func(stat)
            return fn(_self, stat, *args, **kwargs)
        except InvalidStatsNameException:
            log.error('Invalid stat name: %s.', stat, exc_info=True)
            return None
    return cast(T, wrapper)
class AllowListValidator:
    """Class to filter unwanted stats"""
    def __init__(self, allow_list=None):
        if allow_list:
            self.allow_list = tuple(item.strip().lower() for item in allow_list.split(','))
        else:
            self.allow_list = None
    def test(self, stat):
        """Test if stat is in the Allow List"""
        if self.allow_list is not None:
            return stat.strip().lower().startswith(self.allow_list)
        else:
            return True  
class SafeStatsdLogger: