storage: improve block load metrics

[RaduBerinde]

We currently have these two metrics:

    metaBlockBytes = metric.Metadata{
        Name:        "storage.iterator.block-load.bytes",
        Help:        "Bytes loaded by storage engine iterators (possibly cached). See storage.AggregatedIteratorStats for details.",
        Measurement: "Bytes",
        Unit:        metric.Unit_BYTES,
    }
    metaBlockBytesInCache = metric.Metadata{
        Name:        "storage.iterator.block-load.cached-bytes",
        Help:        "Bytes loaded by storage engine iterators from the block cache. See storage.AggregatedIteratorStats for details.",
        Measurement: "Bytes",
        Unit:        metric.Unit_BYTES,
    }

In practice, the most likely figure we would be interested in is the difference between these two, i.e. how many bytes we read. We should replace the total value with the non-cached value.

Jira issue: CRDB-39220

[blathers-crl[bot]]

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

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