**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
"""Timer metric that can be cancelled"""
        if stat and self.allow_list_validator.test(stat):
            tags = tags or []
            return Timer(self.dogstatsd.timer(stat, *args, tags=tags, **kwargs))
        return Timer()
class _Stats(type):
    factory = None
    instance: Optional[StatsLogger] = None
    def __getattr__(cls, name):
        if not cls.instance:
            try:
                cls.instance = cls.factory()
            except (socket.gaierror, ImportError) as e:
                log.error("Could not configure StatsClient: %s, using DummyStatsLogger instead.", e)
                cls.instance = DummyStatsLogger()
        return getattr(cls.instance, name)
    def __init__(cls, *args, **kwargs):
        super().__init__(cls)
        if cls.__class__.factory is None:
            is_datadog_enabled_defined = conf.has_option('metrics', 'statsd_datadog_enabled')
            if is_datadog_enabled_defined and conf.getboolean('metrics', 'statsd_datadog_enabled'):
                cls.__class__.factory = cls.get_dogstatsd_logger
            elif conf.getboolean('metrics', 'statsd_on'):
                cls.__class__.factory = cls.get_statsd_logger
            else:
                cls.__class__.factory = DummyStatsLogger
    @classmethod
    def get_statsd_logger(cls):