fix(metrics): reduce overhead of merge_barrier_alignment_duration

[lmatz]

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What's changed and what's your intention?

Might be the cause of #16914

1. with_label_values only call once instead of each time during poll_next.
2. when metric_level is lower than debug, the metric will not be recorded and there will be no overhead related to this metric at all.

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[kwannoel]

metric_level is lower than debug

How do we set metric_level? Is it done via an env var, similar to RUST_LOG?

[lmatz]

cloud set it by environment variable

[fuyufjh]

Thanks!

[lmatz]

Since it is tested by a nightly pipeline, I will just let it run tonight and see if we can close the issue tomorrow.