[portstat] show BPS, PPS, UTIL rates w/o previous clear

[mykolaf]

Signed-off-by: Mykola Faryma [email protected]

- What I did
Handled sonic-net/sonic-buildimage#2738
Show the BPS, PPS rates even if the counters were not cleared yet.
Both for show interface counters and show interface counters rif
- How I did it
If no counters cache available, calculate the rates over the switch uptime.
- How to verify it

- Previous command output (if the output of a command-line utility has changed)

admin@sonic:~$ show int counters
     IFACE    STATE    RX_OK    RX_BPS    RX_PPS    RX_UTIL    RX_ERR    RX_DRP    RX_OVR    TX_OK    TX_BPS    Tx_PPS    TX_UTIL    TX_ERR
----------  -------  -------  --------  --------  ---------  --------  --------  --------  -------  --------  --------  ---------  --------
 Ethernet0        X          0       N/A       N/A        N/A        0        0       N/A        0       N/A       N/A        N/A         0
 Ethernet1        U          0       N/A       N/A        N/A        0        0       N/A       69       N/A       N/A        N/A         0

- New command output (if the output of a command-line utility has changed)

admin@sonic:~ $ show int counters
     IFACE    STATE    RX_OK      RX_BPS    RX_UTIL    RX_ERR    RX_DRP    RX_OVR    TX_OK      TX_BPS    TX_UTIL    TX_ERR    TX_DRP    TX_OVR
----------  -------  -------  ----------  ---------  --------  --------  --------  -------  ----------  ---------  --------  --------  --------
 Ethernet0        X        0    0.00 B/s      0.00%         0         0       N/A        0    0.00 B/s      0.00%         0         0       N/A
 Ethernet1        U        0    0.00 B/s      0.00%         0         0       N/A       69    7.75 B/s      0.00%         0         0       N/A

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