**Apache Airflow version**: 2.0.1
**Python version**: 3.8
**Cloud provider or hardware configuration**: AWS

**What happened**:
In order to add tags to [Airflow metrics,](https://airflow.apache.org/docs/apache-airflow/stable/logging-monitoring/metrics.html), it's required to set `AIRFLOW__METRICS__STATSD_DATADOG_ENABLED` to `True` and add tags in the `AIRFLOW__METRICS__STATSD_DATADOG_TAGS` variable. We were routing our statsd metrics to Datadog anyway, so this should theoretically have not changed anything other than the addition of any specified tags.
Setting the environment variable `AIRFLOW__METRICS__STATSD_DATADOG_ENABLED` to `True` (along with the other required statsd connection variables) results in the following error, which causes the process to terminate. This is from the scheduler, but this would apply anywhere that `Stats.timer()` is being called.
```
AttributeError: 'DogStatsd' object has no attribute 'timer'
return Timer(self.dogstatsd.timer(stat, *args, tags=tags, **kwargs))
File "/usr/local/lib/python3.8/site-packages/airflow/stats.py", line 345, in timer
return fn(_self, stat, *args, **kwargs)
File "/usr/local/lib/python3.8/site-packages/airflow/stats.py", line 233, in wrapper
timer = Stats.timer('scheduler.critical_section_duration')
File "/usr/local/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 1538, in _do_scheduling
num_queued_tis = self._do_scheduling(session)
File "/usr/local/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 1382, in _run_scheduler_loop
self._run_scheduler_loop()
File "/usr/local/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 1280, in _execute
Traceback (most recent call last):
```

**What you expected to happen**:
The same default Airflow metrics get sent by connecting to datadog, tagged with the metrics specified in `AIRFLOW__METRICS__STATSD_DATADOG_TAGS`.

**What do you think went wrong?**:
There is a bug in the implementation of the `Timer` method of `SafeDogStatsdLogger`. https://github.com/apache/airflow/blob/master/airflow/stats.py#L341-L347
`DogStatsd` has no method called `timer`. Instead it should be `timed`: https://datadogpy.readthedocs.io/en/latest/#datadog.dogstatsd.base.DogStatsd.timed

**How to reproduce it**:
Set the environment variables (or their respective config values) `AIRFLOW__METRICS__STATSD_ON`, `AIRFLOW__METRICS__STATSD_HOST`, `AIRFLOW__METRICS__STATSD_PORT`, and then set `AIRFLOW__METRICS__STATSD_DATADOG_ENABLED` to `True` and start up Airflow.


**Anything else we need to know**:
How often does this problem occur? Every time


airflow/stats.py
"""Decrement stat"""
        if self.allow_list_validator.test(stat):
            return self.statsd.decr(stat, count, rate)
        return None
    @validate_stat
    def gauge(self, stat, value, rate=1, delta=False):
        """Gauge stat"""
        if self.allow_list_validator.test(stat):
            return self.statsd.gauge(stat, value, rate, delta)
        return None
    @validate_stat
    def timing(self, stat, dt):
        """Stats timing"""
        if self.allow_list_validator.test(stat):
            return self.statsd.timing(stat, dt)
        return None
    @validate_stat
    def timer(self, stat=None, *args, **kwargs):
        """Timer metric that can be cancelled"""
        if stat and self.allow_list_validator.test(stat):
            return Timer(self.statsd.timer(stat, *args, **kwargs))
        return Timer()
class SafeDogStatsdLogger:
    """DogStatsd Logger"""
    def __init__(self, dogstatsd_client, allow_list_validator=AllowListValidator()):
        self.dogstatsd = dogstatsd_client
        self.allow_list_validator = allow_list_validator
    @validate_stat
    def incr(self, stat, count=1, rate=1, tags=None):