Metric `batch_executor_token_wait_duration` is incorrectly recorded

[zyguan]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

The buckets declared at metrics/tikvclient.go has a unit of nano second, however, it observed in the seconds at 2pc.go.

2. What did you expect to see? (Required)

metrics.TiKVTokenWaitDuration.Observe(batchExe.tokenWaitDuration)

3. What did you see instead (Required)

metrics.TiKVTokenWaitDuration.Observe(batchExe.tokenWaitDuration.Seconds())

4. What is your TiDB version? (Required)

master

[cfzjywxk]

@zyguan
#17268 did some fix on this metric and changed the observing value to seconds instead of nanoseconds.

[cfzjywxk]

/cc @lysu

[ti-srebot]

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