Fix exception when attempting to write a datetime to db (sonic-net#5467)

redis-py 3.0 used in master branch only accepts user data as bytes,
strings or numbers (ints, longs and floats). Attempting to specify a key
or a value as any other type will raise a DataError exception.
This PR address the issue bt converting datetime to str

files/image_config/procdockerstatsd/procdockerstatsd

@@ -184,9 +184,9 @@ class ProcDockerStats(daemon_base.DaemonBase):
             self.update_dockerstats_command()
             datetimeobj = datetime.now()
             # Adding key to store latest update time.
-            self.update_state_db('DOCKER_STATS|LastUpdateTime', 'lastupdate', datetimeobj)
+            self.update_state_db('DOCKER_STATS|LastUpdateTime', 'lastupdate', str(datetimeobj))
             self.update_processstats_command()
-            self.update_state_db('PROCESS_STATS|LastUpdateTime', 'lastupdate', datetimeobj)
+            self.update_state_db('PROCESS_STATS|LastUpdateTime', 'lastupdate', str(datetimeobj))
 
             # Data need to be updated every 2 mins. hence adding delay of 120 seconds
             time.sleep(120)