**Apache Airflow version**: 2.0.2

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): N/A

**Environment**: N/A

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:
* `dag_processing.last_runtime.*` - In version 1.10.6 [UPDATING.md](https://github.com/apache/airflow/blob/master/UPDATING.md#airflow-1106) it was indicated that this metrics will be removed in 2.0. It was not removed from the metrics documentation. Also the metrics documentation doesn't mention it supposed to be removed/deprecated, it's documented as a gauge but it is actually a timer (reported https://github.com/apache/airflow/issues/10091). 
* `dag_processing.processor_timeouts`: documented as a guage but it is actually a counter. Again from https://github.com/apache/airflow/issues/10091. 
* `dag_file_processor_timeouts` - indicated as supposed to be removed in 2.0, was not removed from [code](https://github.com/apache/airflow/blob/37d549/airflow/utils/dag_processing.py#L1169) but removed from docs. 
* Would be nice if documentation of 1.10.15 indicated the deprecated metrics more clearly, not only in `UPDATING.md`.

**What you expected to happen**:

* The Metrics page should document all metrics being emitted by Airflow.
* The Metrics page should correctly document the type of the metric.

**How to reproduce it**:
<!---

As minimally and precisely as possible. Keep in mind we do not have access to your cluster or dags.

If you are using kubernetes, please attempt to recreate the issue using minikube or kind.

## Install minikube/kind

- Minikube https://minikube.sigs.k8s.io/docs/start/
- Kind https://kind.sigs.k8s.io/docs/user/quick-start/

If this is a UI bug, please provide a screenshot of the bug or a link to a youtube video of the bug in action

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--->

Check official [Metrics Docs](https://airflow.apache.org/docs/apache-airflow/stable/logging-monitoring/metrics.html?highlight=metrics#)

**Anything else we need to know**:

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from setproctitle import setproctitle
from sqlalchemy.orm import Session
from tabulate import tabulate
import airflow.models
from airflow.callbacks.callback_requests import CallbackRequest, SlaCallbackRequest
from airflow.configuration import conf
from airflow.dag_processing.processor import DagFileProcessorProcess
from airflow.models import errors
from airflow.models.dag import DagModel
from airflow.models.dagwarning import DagWarning
from airflow.models.db_callback_request import DbCallbackRequest
from airflow.models.serialized_dag import SerializedDagModel
from airflow.stats import Stats
from airflow.utils import timezone
from airflow.utils.file import list_py_file_paths, might_contain_dag
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.mixins import MultiprocessingStartMethodMixin
from airflow.utils.net import get_hostname
from airflow.utils.process_utils import (
    kill_child_processes_by_pids,
    reap_process_group,
    set_new_process_group,
)
from airflow.utils.session import NEW_SESSION, provide_session
from airflow.utils.sqlalchemy import prohibit_commit, skip_locked, with_row_locks
class DagParsingStat(NamedTuple):
    """Information on processing progress"""
    done: bool
    all_files_processed: bool
class DagFileStat(NamedTuple):