### Apache Airflow version

2.4.1

### What happened

When there are large number of dag files, lets say 10K and each takes sometime to parse,  dag_parser doesnt pick any newly created files till all 10k files are finished parsing

`if not self._file_path_queue:
                self.emit_metrics()
                self.prepare_file_path_queue()`

Above logic only adds new files to queue when queue is empty

### What you think should happen instead

Every loop of dag processor should pick new files and add into file for parsing queue

### How to reproduce

_No response_

### Operating System

All

### Versions of Apache Airflow Providers

_No response_

### Deployment

Docker-Compose

### Deployment details

_No response_

### Anything else

_No response_

### Are you willing to submit PR?

- [X] 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)

"""
        Should only be used when launched DAG file processor manager in sync mode.
        Send agent heartbeat signal to the manager, requesting that it runs one
        processing "loop".
        Call wait_until_finished to ensure that any launched processors have
        finished before continuing
        """
        if not self._parent_signal_conn or not self._process:
            raise ValueError("Process not started.")
        if not self._process.is_alive():
            return
        try:
            self._parent_signal_conn.send(DagParsingSignal.AGENT_RUN_ONCE)
        except ConnectionError:
            
            
            pass
    def get_callbacks_pipe(self) -> MultiprocessingConnection: