testutil: add native histogram assertion helpers

[pedrampdd]

Fixes #1824.

cc @kakkoyun — picking up the proposal from the issue (no PR had been opened for it yet).

What

GatherAndCompare compares metrics via the text exposition format, which carries no representation of native (exponential) histogram buckets — only count and sum. So there is currently no way in testutil to assert on a native histogram's contents. This adds four Gatherer-based helpers:

func GatherAndAssertNativeHistogramExists(g prometheus.Gatherer, name string, labels prometheus.Labels) error
func GatherAndAssertNativeHistogramCount(g prometheus.Gatherer, name string, labels prometheus.Labels, want uint64) error
func GatherAndAssertNativeHistogramSum(g prometheus.Gatherer, name string, labels prometheus.Labels, want, tolerance float64) error
func GatherAndAssertNativeHistogramIntervalCount(g prometheus.Gatherer, name string, labels prometheus.Labels, lower, upper, want, tolerance float64) error

These cover the four capabilities the issue asks for: that a native histogram was gathered, its count, its sum, and the number of observations in an interval.

Design notes

• Naming follows the existing GatherAndCompare / GatherAndCount family. (The issue floated AssertNativeHistogram*; glad to rename if you prefer.)
• Labels selector — each helper takes a prometheus.Labels to pick a series, so it works with HistogramVec. nil means "no filter", but it still errors if the name resolves to more than one series, so the selected series is unambiguous.
• Interval count reconstructs the sparse buckets from the spans + deltas (integer histograms) or absolute counts (float histograms), then estimates the count in the half-open (lower, upper] interval via cumulative rank with log2 interpolation for regular buckets and linear interpolation for the zero bucket — the same approach as PromQL histogram_fraction. Because buckets are exponential it is an estimate, hence the tolerance; aligning the bounds to bucket edges gives an exact result. Total count, by contrast, is exact (no tolerance).
• Scope — Gatherer-only for now (no CollectAndAssert* siblings) to keep the surface small; easy to add the symmetry with CollectAndCompare if wanted.

Tests

White-box tests build real native histograms via NewHistogram / NewHistogramVec and assert through the public API, plus a stub Gatherer to exercise the float-histogram path (no public client API produces those). Covered: integer and float histograms; HistogramVec label selection, ambiguity, and no-match; classic-histogram and non-histogram rejection; zero observations; an observation at exactly 0; negative observations; and both boundary-aligned (exact) and straddling (interpolated — quarter/half-bucket and negative-bucket) interval counts. go test -race is green; gofmt and go vet are clean.