Add metric for the number of merge buffers per query

[TessaIO]

Description

Based on the discussion here, we want to have a view on the count of merge buffers used by different queries, and not a point-in-time metric for that. That would help us to estimate how many merge buffers we need in total to optimize the cluster size.

Motivation

• Mentioned above

[TessaIO]

cc @gianm

[github-actions]

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[zhimin-z]

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