storage: pebble category metrics are initialized too late

[RaduBerinde]

We initialize the Pebble category stats lazily here: https://github.com/cockroachdb/cockroach/blob/master/pkg/kv/kvserver/metrics.go#L4142

This is not a supported pattern. It interacts badly with the metrics.html generation (which uses cockroach gen metric-list), resulting in non-deterministic results.

This issue tracks fixing this. We should replace the category string with an enum value. Pebble can export a RegisterCategory("name") that we can call from init code. Then newStoreMetrics can enumerate all the categories. This will also get rid of pebbleCategoryIterMetricsContainer.metricsMap (we can use a simple immutable array).

Jira issue: CRDB-43648

[blathers-crl[bot]]

Hi @RaduBerinde, please add branch-* labels to identify which branch(es) this C-bug affects.

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