prometheus: land the review fixes #232 merged without

[sathvik09]

Follow-up to #232, which was merged without its review fixes.

The three [high] findings on that PR were each answered in their review thread, but the code implementing those answers was never committed — the branch's last commit (83be4c5) predates the review, and the merge took the pre-review tree. So main currently carries the discussion without the change. This PR carries the change.

Nothing here is new scope: it is exactly what those threads say was done.

What #232 merged without

Review thread	Still wrong on main	Fixed in
DefaultBuckets fallback puts the library's own byte histograms on seconds buckets	httpstats/metrics.go:17 and netstats/conn.go:17 still register "http.message:body.bytes" — keys nothing looks up since b45dd38 (Aug 2019) changed the separator. Those byte histograms fall through to the seconds-scale prometheus.DefaultBuckets: eleven boundaries no byte count can reach.	852585a
_total suffixing double-suffixes dotted names and merges distinct fields	prometheus/metric.go:166 still tests the received field name, so Incr("requests.total") publishes app_requests_total_total, and a counter hits beside a field hits_total puts two samples with identical labels under one # TYPE line.	496391f
Sub-engines still need their own SetBuckets call	engine.go and README.md:221 claim a WithPrefix child resolves an ancestor's registration. It does not.	852585a

The self-correction on the second thread — that exposedName consulted the live entries, so a counter renamed itself once its colliding sibling expired past MetricTimeout — is included in 496391f.

Changes

852585a — stats: make histogram bucket keys expressible and resolvable

HistogramBuckets.Set splits on the last ., so a field carrying a . of its own (body.bytes, header.size, rtt.seconds) is unreachable through it — no string argument yields {Measure: "http.message", Field: "body.bytes"}. A second mismatch sat behind the first: the lookup uses the measure name after the engine prefix is attached, and a package registering from init() cannot know that prefix.

• SetKey(Key, ...) takes the Measure and Field halves directly.
• SetUnprefixed(Key, ...) registers a measure named without the prefix an engine will add to it.
• Lookup(measure, field) resolves those by dropping leading segments after an exact lookup fails. Exact registrations always win, and only SetUnprefixed registrations match that way — suffix-matching everything cannot tell a derived measure from an unrelated one ending the same way.
• Engine.SetBuckets now splits the name itself and registers through SetKey, so an ancestor can register for a sub-engine by naming the path to it.
• Set is unchanged, and the otlp handler, which reads the registry map directly, is untouched. procstats registers "go.memstats:gc_pause.seconds" for fields declared type:"gauge", so that entry stays inert and is left alone.

496391f — prometheus: decide the _total suffix on the name as exposed

hasTotalSuffix runs the test on the rendered name: a field is joined to its scope by an _, and . renders as _. Where the suffix would collide with a sibling field, it is dropped rather than the two being merged — losing a naming convention beats losing a series. exposedName decides that from every name the store has held, which cleanup deliberately does not prune, so the choice cannot flip mid-process. That set is bounded by the program's metric vocabulary; label cardinality lives in metricEntry.states, not here.

8e9adf7 — docs: record the follow-up fixes in the v5.11.0 entry

v5.11.0 is not tagged yet, so the notes are amended in place rather than opening a v5.12.0 section. Two claims from #232 are corrected: histograms whose registered boundaries already ended in math.Inf(+1) were always evaluable, and SetBuckets does not let a sub-engine inherit an ancestor's buckets.

Testing

• go build ./..., go vet ./..., go test ./... — all packages pass, including otlp.
• Both intermediate commits tested independently (./, ./prometheus/..., ./httpstats/..., ./netstats/...), so the branch stays bisectable.
• gofmt -l . clean.
• New tests: TestHistogramBucketsSetKeyDottedField, TestHistogramBucketsLookupUnprefixed, TestHistogramBucketsLookupExactWins, TestHistogramBucketsSuffixMatchingIsOptIn, TestEngineSetBucketsFromAncestor, TestHTTPStatsBucketsReachTheHandler, TestCounterTotalSuffixUsesRenderedName, TestCounterTotalSuffixDottedName, TestCounterTotalSuffixCollision, TestCounterTotalSuffixCollisionSurvivesCleanup.

Release

v5.11.0 is unreleased, so this lands before the tag and no released API changes. Once merged: date the HISTORY.md heading, then make release (bump_version --tag-prefix=v minor version/version.go) and push with --follow-tags.

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