KAFKA-7223: document suppression buffer metrics

[vvcephei]

Document the new metrics added in #5795

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[vvcephei]

Excerpt of processor node metrics:

New intro for state store metrics:

Relevant new metrics:

Question: It doesn't seem like the suppression metrics fit in that well with the other state store metrics. Should I just make it a separate section, and possibly merge them in the future when there is some overlap?

[vvcephei]

@guozhangwang @bbejeck , can you take a look at this when you have a chance?

[guozhangwang]

@vvcephei let's use a separate section for suppression buffers.

[bbejeck]

LGTM modulo @guozhangwang's comment about a separate section

[vvcephei]

Ok @guozhangwang @bbejeck ,

I've moved the buffer metrics to another table:

Also removed it from the list of store types:

Lastly, I happened to notice that there's some markdown syntax that got into this file, so I've fixed it by replacing double-backticks with <code> elements.

[guozhangwang]

LGTM!