### 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
"""Decrement stat"""
        if self.allow_list_validator.test(stat):
            tags = tags or []
            return self.dogstatsd.decrement(metric=stat, value=count, tags=tags, sample_rate=rate)
        return None
    @validate_stat
    def gauge(self, stat, value, rate=1, delta=False, tags=None):
        """Gauge stat"""
        if self.allow_list_validator.test(stat):
            tags = tags or []
            return self.dogstatsd.gauge(metric=stat, value=value, tags=tags, sample_rate=rate)
        return None
    @validate_stat
    def timing(self, stat, dt, tags=None):
        """Stats timing"""
        if self.allow_list_validator.test(stat):
            tags = tags or []
            return self.dogstatsd.timing(metric=stat, value=dt, tags=tags)
        return None
    @validate_stat
    def timer(self, stat=None, *args, tags=None, **kwargs):
        """Timer metric that can be cancelled"""
        if stat and self.allow_list_validator.test(stat):
            tags = tags or []
            return Timer(self.dogstatsd.timed(stat, *args, tags=tags, **kwargs))
        return Timer()
class _Stats(type):