kvserver: add storage time-series metrics for level size and score

[nicktrav]

Currently, the only way to infer the compaction score and heuristics is to use the LSM printout from the logs (emitted once every ten minutes), or to call the /debug/lsm endpoint manually, and track values over time. This makes it difficult to debug issues retroactively.

Add two new sets of per-LSM-level time-series metrics for level size and level score. These new metrics have names of the form storage.$LEVEL-level-{size,score}.

Add an additional enum value for metrics that are "unitless". For example, a "score".

Closes #88415.

Release note (ops change): Adds two new sets of per-LSM-level time-series metrics, one for level size and another for level score. These metrics are of the form storage.$LEVEL-level-{size,score}.

[cockroach-teamcity]

This change is 

[nicktrav]

Example of how these can be used:

[nicktrav]

TFTRs!

bors r=sumeerbhola,jbowens

[craig[bot]]

Build succeeded:

• Bazel Essential CI (Cockroach)

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error creating merge commit from d41cce07e6649d0f34cbbb03dc25eb9156055b2c to blathers/backport-release-22.1-88504: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

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Backport to branch 22.1.x failed. See errors above.

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