### 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
_start_time: Optional[int]
    duration: Optional[int]
    def __init__(self, real_timer=None):
        self.real_timer = real_timer
    def __enter__(self):
        return self.start()
    def __exit__(self, exc_type, exc_value, traceback):
        self.stop()
    def start(self):
        """Start the timer"""
        if self.real_timer:
            self.real_timer.start()
        self._start_time = time.perf_counter()
        return self
    def stop(self, send=True):
        """Stop the timer, and optionally send it to stats backend"""
        self.duration = time.perf_counter() - self._start_time
        if send and self.real_timer:
            self.real_timer.stop()
class DummyStatsLogger:
    """If no StatsLogger is configured, DummyStatsLogger is used as a fallback"""
    @classmethod
    def incr(cls, stat, count=1, rate=1):
        """Increment stat"""
    @classmethod
    def decr(cls, stat, count=1, rate=1):
        """Decrement stat"""
    @classmethod
    def gauge(cls, stat, value, rate=1, delta=False):