db: expose interval metrics as cumulative metrics

[jbowens]

InternalIntervalMetrics exposes metrics computed between successive calls to InternalIntervalMetrics. These metrics can be converted to cumulative metrics and exposed on the Metrics struct. This will allow multiple callers to access the metrics (required for cockroachdb/cockroach#85755) and optionally compute deltas between their own successive calls, if necessary.

Temporarily, I think we can duplicate these metrics in Metrics(). Once the existing CockroachDB admission control has been switched over to use the cumulative versions, we can remove InternalIntervalMetrics.

Some metrics will require a little adjustment to be exposed as cumulative values:

SyncQueueUtilization and PendingBufferUtilization — We should wrap these metrics' underlying GaugeSampleMetric with a simple interface that exposes a Subtract method and a Utilization method that will divide by the appropriate constant.

The ThroughputMetrics can be returned as-is, although we might also want to add a Subtract method to this type.