Allow "/" in metrics validator (apache#42934)

* Allow "/" to avoid ERROR - Invalid stat name: dag_processing.processes,file_path=/mnt/c

* Add UT

* Reformat

airflow/metrics/validators.py

@@ -46,7 +46,7 @@ class MetricNameLengthExemptionWarning(Warning):
 
 # Only characters in the character set are considered valid
 # for the stat_name if stat_name_default_handler is used.
-ALLOWED_CHARACTERS = frozenset(string.ascii_letters + string.digits + "_.-")
+ALLOWED_CHARACTERS = frozenset(string.ascii_letters + string.digits + "_.-/")
 
 # The following set contains existing metrics whose names are too long for
 # OpenTelemetry and should be deprecated over time. This is implemented to

tests/core/test_stats.py

@@ -472,6 +472,10 @@ def test_increment_counter_with_tags(self):
         )
         self.statsd_client.incr.assert_called_once_with("test_stats_run.delay,key0=0,key1=val1", 1, 1)
 
+    def test_increment_counter_with_tags_and_forward_slash(self):
+        self.stats.incr("test_stats_run.dag", tags={"path": "/some/path/dag.py"})
+        self.statsd_client.incr.assert_called_once_with("test_stats_run.dag,path=/some/path/dag.py", 1, 1)
+
     def test_does_not_increment_counter_drops_invalid_tags(self):
         self.stats.incr(
             "test_stats_run.delay",