Make semantic convention selection OpenTelemetry instance-scoped

[trask]

Is your feature request related to a problem? Please describe.

SemconvStability resolves database, messaging, RPC, code, and service-peer modes once from GlobalOpenTelemetry. Instrumentation built with a different ExtendedOpenTelemetry instance can therefore ignore that instance's declarative semconv configuration.

Describe the solution you'd like

Resolve semconv mode from the supplied OpenTelemetry instance and apply the result consistently to attribute extraction, span and metric names, metric units and keys, and schema URLs.

Describe alternatives you've considered

Keeping global selection preserves internal consistency but does not support instance-scoped declarative configuration. Updating schema URLs alone can mislabel telemetry whose attributes still use the global selection.

Additional context

Identified while reviewing #20127.

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