Fix labelled heartbeat metrics reporting 0 with a buffered meter

[Mahnoor-Zaffar]

What & Why

When a worker is configured with a buffered metric meter (e.g. the Python SDK's MetricBuffer), every labelled worker-heartbeat metric — all slot counts and all poller counts — is reported to the server as 0, even though the worker is demonstrably serving work. Unlabelled heartbeat metrics (total_processed_tasks, etc.) are unaffected.

Root cause: Worker heartbeats do not query slot suppliers/pollers directly. They read AtomicU64s that the metrics pipeline writes as a side effect via gauge_with_in_memory → HeartbeatMetricType::WithLabel. The label lookup in label_value_from_attributes only handled Prometheus, OTel, and NoOp; MetricAttributes::Buffer fell through to None, so the atomic was never written and kept its #[derive(Default)] value of 0.

The buffered handle is an opaque LazyRef<Arc<dyn CustomMetricAttributes>> (a Python-held PyDict in the Python bridge), which core cannot read without taking a GIL — exactly what the buffered design avoids. So core retains the Vec<MetricKeyValue> it already has in hand at attribute-creation time instead.

Changes

• temporalio-common: MetricAttributes::Buffer now also carries a core-side kvs: Arc<Vec<MetricKeyValue>> snapshot; label_value_from_attributes resolves labels from it.
• temporalio-sdk-core: MetricsCallBuffer::new_attributes/extend_attributes capture the kvs (merging last-wins on extend); BufferInstrument::send destructures the new shape. The MetricEvent protocol is unchanged, so language bridges compile untouched.
• Tests: unit regression covering buffered label resolution at both the HeartbeatMetricType and MetricsContext/in_memory_meter levels, plus a buffered backing added to the worker-heartbeat integration test.
• Changelog entry under Unreleased.

Validation

• cargo test -p temporalio-common --features core-telemetry-bridge --lib in_memory_attributes_provide_label_values — pass
• cargo test -p temporalio-sdk-core --lib telemetry::metrics — pass (incl. new regression; confirmed it fails without the fix)
• Integration test target compiles; clippy clean for these changes; cargo +nightly fmt --check clean.

Fixes the Python SDK issue reported in temporalio/sdk-python#1817.

[CLAassistant]

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[Sushisource]

Overall I think this makes sense but we shouldn't need to be pulling in any updates from api_upstream in this PR