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What's changed and what's your intention?

Previously, we track the total memory usage of all imm. However, some imm may have already been flushed into sst, but still held by some ongoing iter, which means the current total imm usage is the combination of uploader imms and some orphan imms. In this PR, we introduce a new metrics that only track the total imm usage of uploader imm. We add grafana panel to see the total size of unflushed imm and orphan imm.

In uploader, imm are wrapped by ImmUploader struct, which does not implement Clone to ensure that each imm only exist in place in uploader. When ImmUploader is constructed, it will increment the uploader imm total size, and on Drop, it will decrement the size.

We won't store the instance id in imm any more, because in all usages, we can explicitly know the instance id that an imm belongs to.

Checklist

• I have written necessary rustdoc comments
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Release note

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