Allow primitive OTEL delta ingestion to be enabled

[fionaliao]

What this PR does

This PR introduces the -distributor.otel-native-delta-ingestion flag (and corresponding per-tenant setting), which enables primitive OTEL delta metrics ingestion via the OTLP endpoint. This feature was implemented in Prometheus in prometheus/prometheus#16360. This PR allows Mimir users to enable this feature too.

As per the Prometheus PR:

This allows otlp metrics with delta temporality to be ingested and stored as-is, with metric type unknown. To get "increase" or "rate", sum_over_time(metric[<interval>]) (/ <interval>) can be used.

This is the first step towards implementing prometheus/proposals#48. That proposal has additional suggestions around type-aware functions and making the rate() and increase() functions work for deltas too. However, there are some questions around the best way to do querying over deltas, so having this simple implementation without changing any PromQL functions allow us to get some form of delta ingestion out there gather some feedback to decide the best way to go further.

Which issue(s) this PR fixes or relates to

An initial PR for #10439

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[github-actions]

💻 Deploy preview deleted.

[tacole02]

Thanks, @fionaliao !

[Copilot]

Pull Request Overview

This PR adds an experimental flag to ingest raw OTLP delta metrics without conversion.

• Introduce OTelNativeDeltaIngestion in limits, overrides, handler, and converter
• Wire the new flag through tests, CLI help, JSON descriptor, and documentation
• Add TestOTelDeltaIngestion to validate delta behavior

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/util/validation/limits.go	Add OTelNativeDeltaIngestion field and register the CLI flag
pkg/distributor/push_test.go	Add mock implementation for OTelNativeDeltaIngestion
pkg/distributor/otel.go	Extend OTLPHandlerLimits and conversionOptions to include delta flag
pkg/distributor/otel_test.go	New TestOTelDeltaIngestion covering allowed and disallowed cases
docs/sources/mimir/configure/configuration-parameters/index.md	Document the otel_native_delta_ingestion parameter
docs/sources/mimir/configure/about-versioning.md	Add experimental feature entry for delta ingestion
cmd/mimir/help-all.txt.tmpl	Add CLI help entry for -distributor.otel-native-delta-ingestion
cmd/mimir/config-descriptor.json	Add config descriptor for otel_native_delta_ingestion
CHANGELOG.md	Log the new experimental feature under [FEATURE]

Comments suppressed due to low confidence (2)

docs/sources/mimir/configure/about-versioning.md:119

• The backticked flag is missing its leading hyphen. It should be -distributor.otel-native-delta-ingestion to match other entries.

    - `distributor.otel-native-delta-ingestion`

cmd/mimir/help-all.txt.tmpl:1428

• Align this flag's indentation with other entries (use a leading tab) so the CLI help output remains consistently formatted.

  -distributor.otel-native-delta-ingestion

[aknuds1]

LGTM