[counter] Clear counter table when dhcpmon init (#14) (#16)

Cherry-pick from #14

Why I did it
Fix issue sonic-net/sonic-buildimage#15047
dhcp counter didn't restore after config reload

Work item tracking
Microsoft ADO (number only): 26270786
How I did it
Clear all counters when dhcpmon start
Remove clear counter in initialize_counter since they are cleared in previous
How to verify it
Build debian packet and install in DUT do manually testing
Run dhcp_relay test, all passed

src/dhcp_device.cpp

@@ -194,6 +194,25 @@ void update_vlan_mapping(std::shared_ptr<swss::DBConnector> db_conn) {
     }
 }
 
+
+/**
+ * @code clear_counter(std::shared_ptr<swss::DBConnector> state_db);
+ *
+ * @brief Clear all counter
+ *
+ * @param state_db      state_db connector pointer
+ *
+ */
+void clear_counter(std::shared_ptr<swss::DBConnector> state_db) {
+    std::string match_pattern = DB_COUNTER_TABLE + std::string("*");
+    auto keys = state_db->keys(match_pattern);
+
+    for (auto &itr : keys) {
+        state_db->del(itr);
+    }
+}
+
+
 /** update ethernet interface to port-channel map
  *  PORTCHANNEL_MEMBER|PortChannel101|Ethernet112
  */
@@ -265,7 +284,6 @@ void initialize_db_counters(std::string &ifname)
 {
     auto table_name = DB_COUNTER_TABLE + ifname;
     auto init_value = gen_counter_json_str(db_counter);
-    mStateDbPtr->del(table_name);
     mStateDbPtr->hset(table_name, "RX", init_value);
     mStateDbPtr->hset(table_name, "TX", init_value);
 }
@@ -924,6 +942,7 @@ int dhcp_device_start_capture(size_t snaplen, struct event_base *base, in_addr_t
 
         init_recv_buffers(snaplen);
 
+        clear_counter(mStateDbPtr);
         update_vlan_mapping(mConfigDbPtr);
         update_portchannel_mapping(mConfigDbPtr);
         update_mgmt_mapping();