[ISSUE #5026]fix MetricsHttpAgent metrics twice

[shalk]

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fix #5026

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[KomachiSion]

I check the code of Histogram.Timer
the close method will call observeDuration.
your fix is ok. But I think use only close maybe better.
Beacuse close is an interface method but observeDuration is a class method.

[KomachiSion]

I check the PR again, and find that this PR is for Nacos1.X. In Nacos1.X, nacos-client is compile by 1.6 jdk. So use observeDuration is right.