isisd: Add PDU drop counter to "show isis summary"

isisd: Add PDU drop counter to "show isis summary"

Adding a new drop counters section to "show isis summary".

New output:

Drop counters per PDU type:
  P2P IIH: <count>
  L2 LSP: <count>
  L2 CSNP: <count>
  L2 PSNP: <count>
  ...

Before:
r1# show isis summary
vrf             : default
Process Id      : 972
System Id       : 0000.0000.0001
Up time         : 00:00:48 ago
Number of areas : 1
Area TE:
  Net: 49.0000.0000.0000.0001.00
  TX counters per PDU type:
    P2P IIH: 36
     L2 LSP: 8
    L2 CSNP: 12
    L2 PSNP: 11
  RX counters per PDU type:
    P2P IIH: 37
     L2 LSP: 17
    L2 CSNP: 12
    L2 PSNP: 6
Advertise high metrics: Disabled
...

After:
r1# show isis summary
vrf             : default
Process Id      : 972
System Id       : 0000.0000.0001
Up time         : 00:00:19 ago
Number of areas : 1
Area TE:
  Net: 49.0000.0000.0000.0001.00
  TX counters per PDU type:
    P2P IIH: 16
     L2 LSP: 2
    L2 CSNP: 4
    L2 PSNP: 6
   LSP RXMT: 0
  RX counters per PDU type:
    P2P IIH: 16
     L2 LSP: 5
    L2 CSNP: 4
    L2 PSNP: 2
  Drop counters per PDU type:
    P2P IIH: 2
  Advertise high metrics: Disabled
...

Signed-off-by: Isabella de Leon <ideleon@microsoft.com>