[show] add support for muxcable metrics

[vdahiya12]

Signed-off-by: vaibhav-dahiya vdahiya@microsoft.com

What I did

Added support for show muxcable metrics. This essentially records what events came to different modules per se
for toggling the mux from one state to another.
for example

admin@sonic$ show muxcable metrics Ethernet0 --json

{
"linkmgrd_switch_active_start": "2021-May-13 10:00:21.420898",
"linkmgrd_switch_standby_end": "2021-May-13 10:01:15.696728",
"linkmgrd_switch_unknown_end": "2021-May-13 10:00:26.123319",
"xcvrd_switch_standby_end": "2021-May-13 10:01:15.696051",
"xcvrd_switch_standby_start": "2021-May-13 10:01:15.690835"
}

or

admin@sonic:$ show muxcable metrics Ethernet0

PORT EVENT TIME
--------- ---------------------------- ---------------------------
Ethernet0 linkmgrd_switch_active_start 2021-May-13 10:00:21.420898
Ethernet0 linkmgrd_switch_standby_end 2021-May-13 10:01:15.696728
Ethernet0 linkmgrd_switch_unknown_end 2021-May-13 10:00:26.123319
Ethernet0 xcvrd_switch_standby_end 2021-May-13 10:01:15.696051
Ethernet0 xcvrd_switch_standby_start 2021-May-13 10:01:15.690835

How I did it

added changes in show/muxcable.py by reading and publishing the state DB contents for the corresponding table

How to verify it

unit tests are added for verifying.

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

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

[tahmed-dev]

is it possible the same date format as does linkmgrd, yyyy-mmm-dd?

[vdahiya12]

It's actually taken care in the xcvrd now, this version was running on old xcvrd changes, I will update the comment and testcases.

`admin@STR43-0101-0101-01LT0:~$ redis-cli -n 6 hgetall "MUX_METRICS_TABLE|Ethernet0"

1. "linkmgrd_switch_active_start"
2. "2021-May-13 10:00:21.420898"
3. "linkmgrd_switch_unknown_end"
4. "2021-May-13 10:00:26.123319"
5. "xcvrd_switch_standby_start"
6. "2021-May-17 18:15:11.057528"
7. "xcvrd_switch_standby_end"
8. "2021-May-17 18:15:11.062379"
9. "linkmgrd_switch_standby_end"
10. "2021-May-17 18:15:11.062887"`