### 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
"""Type protocol for StatsLogger.timer"""
    def __enter__(self):
        ...
    def __exit__(self, exc_type, exc_value, traceback):
        ...
    def start(self):
        """Start the timer"""
        ...
    def stop(self, send=True):
        """Stop, and (by default) submit the timer to statsd"""
        ...
class StatsLogger(Protocol):
    """This class is only used for TypeChecking (for IDEs, mypy, etc)"""
    @classmethod
    def incr(cls, stat: str, count: int = 1, rate: int = 1) -> None:
        """Increment stat"""
    @classmethod
    def decr(cls, stat: str, count: int = 1, rate: int = 1) -> None:
        """Decrement stat"""
    @classmethod
    def gauge(cls, stat: str, value: float, rate: int = 1, delta: bool = False) -> None:
        """Gauge stat"""
    @classmethod
    def timing(cls, stat: str, dt) -> None:
        """Stats timing"""
    @classmethod
    def timer(cls, *args, **kwargs) -> TimerProtocol:
        """Timer metric that can be cancelled"""
class Timer: