feat: include LD context keys in metric event attributes

[Vadman97]

Summary

• Spread _ldContextKeys (set via LDObserve.setLDContextKeyAttributes(...)) into the OTel attribute bag for recordCount, recordGauge, recordHistogram, and recordUpDownCounter in ObserveSDK. Caller-provided metric.attributes still win; highlight.session_id stays last.
• Same change applied to the integration recordMetric tags in recordGauge so gauge data flowing to integrations carries the same dimensions.

Why

Previously, context.contextKeys.* attrs only flowed to track/identify/spans, so gauge and histogram queries (e.g. web-vital LCP/CLS/INP) could not groupBy pathTemplate, owner, or accountId. Verified that service_name=gonfalon-web emits these metrics at high volume but groupBy context.contextKeys.pathTemplate returns empty groups today.

Test plan

• yarn turbo run build --filter highlight.run
• yarn turbo run lint --filter highlight.run
• yarn turbo run test --filter highlight.run (added two new unit tests in sdk.test.ts; 408 tests pass)
• yarn enforce-size (166KB / 256KB brotli)
• yarn format-check

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Note

Medium Risk
Changes the attribute payload for all emitted metrics, which can affect metric cardinality/cost and downstream dashboards/integrations; logic is straightforward and covered by unit tests.

Overview
LaunchDarkly context key attributes are now attached to emitted metrics. ObserveSDK spreads _ldContextKeys into the OpenTelemetry attribute bag for recordCount, recordGauge, recordHistogram, and recordUpDownCounter, while keeping caller-provided metric.attributes as the override source and preserving highlight.session_id.

For gauges, the same merged attributes are also used when building the integration recordMetric (category, group, and tags), so integrations receive the same dimensions. Adds unit tests to verify LD context keys are included and that caller attributes win on key collisions.

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[ntiner1]

Approve with cardinality check in mind