**Description**


It would be great if the metrics we publish in Airflow change to include TAGs for "variable" parts of those (such as dag_id, task_id, pool name) rather than being part of the metrics.

See here: https://airflow.apache.org/docs/1.10.12/metrics.html, https://airflow.readthedocs.io/en/latest/logging-monitoring/metrics.html  - those variable parts should be tags.

We might consider to change those metrics by default in 2.0 and possibly introduce a backport in 1.10.13 to allow people to migrate.


**Use case / motivation**

Having the variable parts of the metrics makes it really difficult to aggregate the metrics. Seems that statsd python library does not support tags, but there are possible solutions by installing extensions:

https://stackoverflow.com/questions/49852654/sending-statsd-metrics-with-tags-from-python-client

**Related Issues**

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"""Start more processors if we have enough slots and files to process."""
        while self._parallelism - len(self._processors) > 0 and self._file_path_queue:
            file_path = self._file_path_queue.popleft()
            
            if file_path in self._processors.keys():
                continue
            callback_to_execute_for_file = self._callback_to_execute[file_path]
            processor = self._create_process(
                file_path,
                self._pickle_dags,
                self._dag_ids,
                self.get_dag_directory(),
                callback_to_execute_for_file,
            )
            del self._callback_to_execute[file_path]
            Stats.incr("dag_processing.processes", tags={"file_path": file_path, "action": "start"})
            processor.start()
            self.log.debug("Started a process (PID: %s) to generate tasks for %s", processor.pid, file_path)
            self._processors[file_path] = processor
            self.waitables[processor.waitable_handle] = processor
    def add_new_file_path_to_queue(self):
        for file_path in self.file_paths:
            if file_path not in self._file_stats:
                
                self.log.info("Adding new file %s to parsing queue", file_path)
                self._file_stats[file_path] = DagFileStat(
                    num_dags=0, import_errors=0, last_finish_time=None, last_duration=None, run_count=0
                )
                self._file_path_queue.appendleft(file_path)
    def prepare_file_path_queue(self):