Lower the accidental opentelemetry-api floor in the otel extra

[bagowix]

Problem

pyproject.toml:31 declares otel = ["opentelemetry-api>=1.43.0"]. That floor was never a requirement of the code: it arrived as a routine dependency bump (3d664ad, "chore: bump opentelemetry-api from 1.42.1 to 1.43.0 (#7)") that moved the dev pin, the extra's lower bound and the extras-min pin (.github/workflows/ci.yml:105) together. A dev pin says "what we develop against"; an extra's floor says "the oldest version we support" — the bump silently answered the second question with the first.

OTelEventListener uses only metrics.get_meter, Meter.create_histogram and Meter.create_counter (interlock/integrations/otel.py:31-56) — API that has been stable for years. Verified locally: every hook of OTelEventListener runs unchanged on opentelemetry-api==1.20.0 and on 1.35.0.

The cost to adopters is real, because the OpenTelemetry distro/instrumentation line pins the whole stack to one API version. opentelemetry-distro==0.56b0 resolves opentelemetry-sdk==1.35.0 and therefore opentelemetry-api==1.35.0; interlock-cb[otel] then forces a choice between upgrading the entire OTel stack and skipping the extra — for a listener that would have worked fine. Every other extra carries a floor that means something (requests>=2.31.0, httpx>=0.27.0, aiohttp>=3.12.0 for the client-middleware API, litestar>=2.23.0 for NamedDependency); this one does not.

Proposal

• Lower the otel extra floor to the oldest opentelemetry-api the listener actually works on, and pin extras-min to the same version so CI proves it on 3.11. 1.20.0 is verified here; go lower only if the job stays green.
• Keep the dev group on the current version — development continues against the latest.
• Prevent the recurrence: a runtime floor is a compatibility statement, not a dev pin. Either exclude [project.optional-dependencies] from the uv updates in .github/dependabot.yml, or state the rule in CONTRIBUTING.md next to the extras workflow so a floor is only ever raised by an API the code needs.

Acceptance criteria

• The otel extra floor is lowered and .github/workflows/ci.yml pins extras-min to the same version; tests/test_otel.py passes there.
• The dev group keeps the current opentelemetry-api version, so the main matrix still tests the latest.
• The "never raise an extra's floor without an API reason" rule is written down where the next bump will be reviewed.
• Audit the remaining extras' floors in the same PR and note any that are equally accidental.
• CHANGELOG.md entry under [Unreleased] (Changed), phrased for the adopter it unblocks.

Non-goals

• Supporting the pre-stable metrics API or adding compatibility shims: if a floor cannot be met without branching on the OTel version, keep the floor.