fix(opentelemetry): honor MetricReader temporality in MetricProducer, closes #6253

[mohamedramadan14]

Fixes #6253.

@effect/opentelemetry's MetricProducerImpl was hardcoding AggregationTemporality.CUMULATIVE on every produced data point (Counter, UpDownCounter, Frequency, Summary count/sum, Gauge, Histogram). It never consulted the registered MetricReader, so any temporalityPreference (e.g. DELTA) configured on OTLPMetricExporter (or any custom aggregationTemporalitySelector) was silently overridden.

Why this needs a small adaptation of the issue's suggested fix #1

The OTel MetricProducer.collect() interface only receives { timeoutMillis } — not a reader reference. The reader is given to the producer one-way via reader.setMetricProducer(producer). So the producer cannot query selectAggregationTemporality directly during collect() without first remembering the reader at registration time.

What the fix does

• MetricProducerImpl now holds a list of registered MetricReaders, populated in registerProducer.
• collect() calls reader.selectAggregationTemporality(descriptor.type) per produced data point and stamps that on the result. The 5 hardcoded sites (Counter, Gauge, Histogram, Frequency, Summary count/sum) all go through one helper.
• No reader registered → unchanged (CUMULATIVE, matching the OTel default).
• Multiple readers, all agree → use that preference.
• Multiple readers, disagree → use the first reader's preference and emit a single diag.warn(...) (non-fatal, non-breaking).

Why option (1) over option (2)

Option (1) is spec-compliant and requires zero user-side configuration; option (2) would have made users re-state the reader's preference on the producer side.

Tests added (packages/opentelemetry/test/Metrics.test.ts)

• counter honors reader's preferred temporality (DELTA) — the exact issue repro
• gauge honors reader's preferred temporality (DELTA)
• falls back to CUMULATIVE when no reader is registered

All 60 tests in @effect/opentelemetry pass. pnpm check, pnpm build, pnpm docgen all green.

Out of scope (noted as a follow-up)

packages/opentelemetry/src/OtlpMetrics.ts (the non-SDK direct-OTLP snapshot path) has the same hardcoded CUMULATIVE, but it does not use a MetricReader. A follow-up could add a temporality option to OtlpMetrics.make since there is no reader to query there.

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Latest commit: 9571e91

The changes in this PR will be included in the next version bump.

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@effect/opentelemetry	Patch

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