release-22.2: metrics: fix windowed histogram merging approach

[ericharmeling]

Backport 1/1 commits from #104088.

/cc @cockroachdb/release

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Fixes #103814.
Fixes #98266.

This commit updates the windowed histogram merging approach to add the previous window's histogram bucket counts and sample count to those of the current one. As a result of this change, the histograms will no longer report under-sampled quantile values, and timeseries metrics-derived charts (e.g., the quantile-based SQL service latency charts on the DB console's Metrics page) will more accurately display metrics.

Release note (bug fix): Updated the histogram window merge calculation to more accurately interpolate quantile values. This change will result in smoother, more accurate Metrics charts on the DB Console.

Release justification: bug fix

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[dhartunian]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @ericharmeling)

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pkg/util/metric/metric.go line 457 at r1 (raw file):

func (h *Histogram) ValueAtQuantileWindowed(q float64) float64 {
	return ValueAtQuantileWindowed(h.ToPrometheusMetricWindowed().Histogram,
		q)

nit: run linter?

[ericharmeling]

Required CI has passed, and I'm not getting any linting/build/generate errors locally...

[dhartunian]

@ericharmeling if required CI passes, you're good to merge