Add time unit for slow query log document

[shenli]

Change Request

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Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.
   https://docs.pingcap.com/tidb/stable/identify-slow-queries#identify-slow-queries
   In the slow query log, there are several fields about execution time, like:

# Cop_proc_avg: 0.07 Cop_proc_p90: 0.07 Cop_proc_max: 0.07 Cop_proc_addr: 172.16.5.87:20171
# Cop_wait_avg: 0 Cop_wait_p90: 0 Cop_wait_max: 0 Cop_wait_addr: 172.16.5.87:20171
# Cop_backoff_regionMiss_total_times: 200 Cop_backoff_regionMiss_total_time: 0.2 Cop_backoff_regionMiss_max_time: 0.2 Cop_backoff_regionMiss_max_addr: 127.0.0.1 Cop_backoff_regionMiss_avg_time: 0.2 Cop_backoff_regionMiss_p90_time: 0.2
# Cop_backoff_rpcPD_total_times: 200 Cop_backoff_rpcPD_total_time: 0.2 Cop_backoff_rpcPD_max_time: 0.2 Cop_backoff_rpcPD_max_addr: 127.0.0.1 Cop_backoff_rpcPD_avg_time: 0.2 Cop_backoff_rpcPD_p90_time: 0.2
# Cop_backoff_rpcTiKV_total_times: 200 Cop_backoff_rpcTiKV_total_time: 0.2 Cop_backoff_rpcTiKV_max_time: 0.2 Cop_backoff_rpcTiKV_max_addr: 127.0.0.1 Cop_backoff_rpcTiKV_avg_time: 0.2 Cop_backoff_rpcTiKV_p90_time: 0.2

But the descriptions don't show the unit. I can not tell the execution time is second or millisecond.

Cop_proc_avg: The average execution time of cop-tasks.
Cop_proc_p90: The P90 execution time of cop-tasks.
Cop_proc_max: The maximum execution time of cop-tasks.

2. Describe your suggestion or addition.
   Add the time unit for the execution time fields.

3. Provide some reference materials (documents, websites, etc) if you could.

[TomShawn]

@crazycs520 Could you take a look? Thanks!

[crazycs520]

@shenli

Note:
The unit of all the following time fields in the slow query log is "second".

the doc is from https://docs.pingcap.com/tidb/stable/identify-slow-queries#fields-description.

Adding unit for serveral fields of execution time will break compatibility with MySQL slow-log format, also infect the usage of pt-query-digest

[TomShawn]

@shenli What do you think of the answer above?

[github-actions]

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