Update lua script to convert poll_interval to MS

[noaOrMlnx]

What I did
Update the rif_rates/lua script to multiply by 1000 instead of FlexCounter class.
related also to - sonic-sairedis/pull/878
Why I did it
times were not calculated properly.
How I verified it
check that the output of cli and redis is close enough:

admin@sonic:~$ show interfaces counters rif -p 5
The rates are calculated within 5 seconds period
    IFACE    RX_OK     RX_BPS    RX_PPS    RX_ERR    TX_OK    TX_BPS    TX_PPS    TX_ERR
---------  -------  ---------  --------  --------  -------  --------  --------  --------
Ethernet0        5  83.90 B/s    1.00/s         1        0  0.00 B/s    0.00/s         0

127.0.0.1:6379[2]> hgetall RATES:oid:0x6000000000d9d
 1) "SAI_ROUTER_INTERFACE_STAT_IN_OCTETS_last"
 2) "3948"
 3) "SAI_ROUTER_INTERFACE_STAT_IN_PACKETS_last"
 4) "47"
 5) "SAI_ROUTER_INTERFACE_STAT_OUT_OCTETS_last"
 6) "0"
 7) "SAI_ROUTER_INTERFACE_STAT_OUT_PACKETS_last"
 8) "0"
 9) "RX_BPS"
10) "77.634107320096575"
11) "RX_PPS"
12) "0.92421556333448329"
13) "TX_BPS"
14) "0"
15) "TX_PPS"
16) "0"

before the change:

admin@sonic:~$ show interfaces  counters rif -p 5
The rates are calculated within 5 seconds period
     IFACE    RX_OK       RX_BPS     RX_PPS    RX_ERR    TX_OK    TX_BPS    TX_PPS    TX_ERR
----------  -------  -----------  ---------  --------  -------  --------  --------  --------
Ethernet0        0     0.00 B/s     0.00/s         0        0  0.00 B/s    0.00/s         0
Ethernet28   29,894  501.05 KB/s  5964.85/s         1        0  0.00 B/s    0.00/s         0

127.0.0.1:6379[2]> hgetall "RATES:oid:0x6000000000517"
1) "SAI_ROUTER_INTERFACE_STAT_IN_OCTETS_last"
2) "17856552"
3) "SAI_ROUTER_INTERFACE_STAT_IN_PACKETS_last"
4) "212578"
5) "SAI_ROUTER_INTERFACE_STAT_OUT_OCTETS_last"
6) "0"
7) "SAI_ROUTER_INTERFACE_STAT_OUT_PACKETS_last"
8) "0"
9) "RX_BPS"
10) "67.774654124880783"
11) "RX_PPS"
12) "0.80684145386762807"
13) "TX_BPS"
14) "0"
15) "TX_PPS"
16) "0"

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