[AIRFLOW-6897] Simplify DagFileProcessorManager

[mik-laj]

Extract start_new_processes and prepare_file_path_queue
These functions are independent of each other, but as separate methods, they are easier to understand. A good methods name is the best documentation.
I also changed to _heartbeat_count to _no_run because it better describes the role. This variable can increase without affecting heartbeat when DagFileProcessorManager is running in asynchronous mode.q
Move emit_metrics call
It is not related to generating file_path_queue, so it should be called by the caller.
Inline heartbeat method
This function does not make sense, because every time a full loop is performed. It only hides the logic and makes it difficult to understand the code.
Move _kill_timed_out_processors to loop
It is not related to generating the collect_results method, so it should be called by the caller.

Now I have the impression that the sequence of operations is much more easy to understand because it is only found in the start method.

In this PR I do not want to change the sequence of operations. It is only refactoring without functional changes.

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Issue link: AIRFLOW-6897

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[codecov-io]

Codecov Report

Merging #7521 into master will decrease coverage by 0.88%.
The diff coverage is 88.15%.

@@            Coverage Diff             @@
##           master    #7521      +/-   ##
==========================================
- Coverage   86.81%   85.92%   -0.89%     
==========================================
  Files         893      893              
  Lines       42193    42229      +36     
==========================================
- Hits        36629    36285     -344     
- Misses       5564     5944     +380

Impacted Files	Coverage Δ
airflow/bin/cli.py	94.73% <ø> (ø)	⬆️
airflow/cli/commands/dag_command.py	84.71% <100%> (ø)	⬆️
airflow/providers/amazon/aws/operators/sns.py	100% <100%> (ø)	⬆️
airflow/models/dagrun.py	95.81% <66.66%> (-0.76%)	⬇️
airflow/www/views.py	76.19% <76.19%> (-0.1%)	⬇️
airflow/providers/amazon/aws/hooks/sns.py	96.42% <94.11%> (-3.58%)	⬇️
airflow/utils/dag_processing.py	87.95% <96.42%> (+0.02%)	⬆️
...flow/providers/apache/cassandra/hooks/cassandra.py	21.51% <0%> (-72.16%)	⬇️
...w/providers/apache/hive/operators/mysql_to_hive.py	35.84% <0%> (-64.16%)	⬇️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
... and 17 more

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[ashb]

Suggested change

	self._no_run = 0
	self._num_run = 0

Please? Seeing _no_run I expected it to be a boolean/flag variable.

[mik-laj]

I renamed this variable.

[ashb]

Other than changing no -> num this looks good, and removes the confusing of having a "heartbeat" method that is unlike the heartbeat method on Jobs (this one doesn't update a row in the DB like the other).

Pre-emptive approve, but please change the variable name.