[portstat] Add FEC FLR statistics support to port counters

[apoorv-arista]

Add support to view FEC FLR (Frame Loss Ratio) statistics in
port counter statistics.

This includes

• FEC_FLR - observed FLR
• FEC_FLR_PREDICTED - predicted FLR computed by linear regression.

can be verified by viewing fec-stats either by portstat -f or show interfaces counters fec-stats

Feature HLD : port_fec_flr.md

What I did

Added FEC FLR (Frame Loss Ratio) statistics support to port counters display, including both actual and predicted FLR values.

How I did it

Added format_fec_flr() function to format FLR values consistently
Extended header_fec_only to include FEC_FLR and FEC_FLR_PREDICTED columns
Updated RateStats namedtuple to include fec_flr and fec_flr_predicted fields
Modified port statistics collection to retrieve FLR values from CHASSIS_STATE_DB
Updated display formatting to show FLR columns in fec-stats output

How to verify it

Run show interfaces counters fec-stats command and verify the output includes the new FEC_FLR and FEC_FLR_PREDICTED columns with properly formatted values.

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

admin@ctd615:~$ show interfaces counters fec-stats
      IFACE    STATE    FEC_CORR    FEC_UNCORR    FEC_SYMBOL_ERR    FEC_PRE_BER    FEC_POST_BER    FEC_PRE_BER_MAX
-----------  -------  ----------  ------------  ----------------  -------------  --------------    ---------------
 Ethernet0        U           0             0                 0        1.48e-20        0.00e+00           1.78e-16
 Ethernet8        U           0             0                 0        1.98e-19        0.00e+00           1.67e-14
Ethernet16        U           0             0                 0        1.77e-20        0.00e+00           1.37e-13

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

admin@ctd615:~$ show interfaces counters fec-stats
      IFACE    STATE    FEC_CORR    FEC_UNCORR    FEC_SYMBOL_ERR    FEC_PRE_BER    FEC_POST_BER    FEC_PRE_BER_MAX    FEC_FLR    FEC_FLR_PREDICTED
-----------  -------  ----------  ------------  ----------------  -------------  --------------    ---------------  ---------  -------------------
 Ethernet0        U           0             0                 0        1.48e-20        0.00e+00           1.78e-16          0                    0
 Ethernet8        U           0             0                 0        1.98e-19        0.00e+00           1.67e-14          0                    0
Ethernet16        U           0             0                 0        1.77e-20        0.00e+00           1.37e-13          0                    0

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[prgeor]

@apoorv-arista please fix the build errors

Also, update the PR as per the HLD

Can you put the link to the HLD in the PR description?

[prgeor]

@apoorv-arista please resolve the conflict