add show dhcp_relay ipv4 counter entry, fix interface name offset issue (#16507)

Why I did it
Add another cli entry: show dhcp_relay ipv4 counter
Fix get all interface offset issue

Work item tracking
Microsoft ADO (17271822):
How I did it
show dhcp_relay ipv4 counter -i [ifname]
show dhcp4relay_counters counts -i [ifname]

How to verify it
show dhcp4relay_counters counts | more 10
Message Type Ethernet144(RX)

Author: jcaiMR

dockers/docker-dhcp-relay/cli/show/plugins/show_dhcp_relay.py

@@ -56,7 +56,7 @@ def get_interface(self):
         interfaces = []
         for key in self.db.keys(self.db.STATE_DB):
             if DHCPv4_COUNTER_TABLE in key:
-                interfaces.append(key[21:])
+                interfaces.append(key[19:])
         return interfaces
 
     def get_dhcp4relay_msg_count(self, interface, dir):
@@ -274,6 +274,10 @@ def dhcp_relay_ipv4_destination():
 def dhcp_relay_ipv6_destination():
     get_dhcp_relay(DHCP_RELAY, DHCPV6_SERVERS, with_header=True)
 
+@dhcp_relay_ipv4.command("counters")
+@click.option('-i', '--interface', required=False)
+def dhcp_relay_ip4counters(interface):
+    ipv4_counters(interface)
 
 @dhcp_relay_ipv6.command("counters")
 @click.option('-i', '--interface', required=False)