[show priority-group drop counters] Remove backup with cached PG drop counters after 'config reload' (#1679)

- What I did
Explanation how show/clear counters works:

Suppose we got from SDK some count of dropped packets for priority group (№3) per interface (Ehernet4). (for example Ethernet4 PG-3 123 packets)
Stats can be shown via show priority-group drop counters
Then we run sonic-clear priority-group drop counters. This command will save data "Ethernet4 PG-3 123 packets" to backup. It saved in /tmp/dropstat-{user_id} folder
When we run show priority-group drop counters again, script will take data from SDK and data saved from backup. In backup it will find 123. In SDK it may still be number 123 for Ethernet4 (like in backup) or bigger (for example 126.)
Our sonic-clear priority-group drop counters command does not change anything in SDK. It just show difference beetwen SDK data and last saved data.
So if in SDK data is 123, show priority-group drop counters will show 0 (123 minus 123) - data is cleared
If in SDK data is bigger than saved 124, or higher show priority-group drop counters will show 1 (124 minus 123)
FIX
After config reload all counters drop counters data retrieved from SDK become cleared. All counters become zeros.
In this case show priority-group drop counters will show -123 ( 0 (from SDK) minus 123 (saved backup))
So we don't need backup after config reload

- How I did it
remove /tmp/dropstat-{user_id} folders with counters backup

Signed-off-by: Andriy Yurkiv <ayurkiv@nvidia.com>

Author: ayurkiv-nvda

config/main.py

@@ -1253,6 +1253,11 @@ def reload(db, filename, yes, load_sysinfo, no_service_restart, disable_arp_cach
     if multi_asic.is_multi_asic():
         num_cfg_file += num_asic
 
+    # Remove cached PG drop counters data
+    dropstat_dir_prefix = '/tmp/dropstat'
+    command = "rm -rf {}-*".format(dropstat_dir_prefix)
+    clicommon.run_command(command, display_cmd=True)
+
     # If the user give the filename[s], extract the file names.
     if filename is not None:
         cfg_files = filename.split(',')

tests/pgdropstat_test.py

@@ -3,6 +3,7 @@
 
 import show.main as show
 import clear.main as clear
+import config.main as config
 
 from click.testing import CliRunner
 
@@ -62,6 +63,20 @@ def executor(self, clear_before_show):
         assert result.exit_code == 0
         assert result.output == show_output
 
+    def test_show_pg_drop_config_reload(self):
+        runner = CliRunner()
+        self.test_show_pg_drop_clear()
+
+        # simulate 'config reload' to provoke counters recalculation (remove  backup from /tmp folder)
+        result = runner.invoke(config.config.commands["reload"], [ "--no_service_restart",  "-y"])
+
+        print(result.exit_code)
+        print(result.output)
+
+        assert result.exit_code == 0
+
+        self.test_show_pg_drop_show()
+
     @classmethod
     def teardown_class(cls):
         os.environ["PATH"] = os.pathsep.join(os.environ["PATH"].split(os.pathsep)[:-1])