[FEATURE] Add gauge metrics for reading data

[rickyma]

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Describe the feature

Currently, we only have the counter metrics, but not the gauge metrics, which we often care more about. I think we need to know the maximum number of threads reading data at a particular moment, as well as the cumulative size of the data being read at that moment. So we should introduce gauge metrics for reading data.

Motivation

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Describe the solution

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Additional context

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Are you willing to submit PR?

• Yes I am willing to submit a PR!