### 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)

"""Check if the DagFileProcessorManager process is alive, and process any pending messages"""
        if not self._parent_signal_conn:
            raise ValueError("Process not started.")
        
        while self._parent_signal_conn.poll(timeout=0.01):
            try:
                result = self._parent_signal_conn.recv()
            except (EOFError, ConnectionError):
                break
            self._process_message(result)
        
        self._heartbeat_manager()
    def _process_message(self, message):
        self.log.debug("Received message of type %s", type(message).__name__)
        if isinstance(message, DagParsingStat):
            self._sync_metadata(message)
        else:
            raise RuntimeError(f"Unexpected message received of type {type(message).__name__}")
    def _heartbeat_manager(self):