core,opentelemetry: Fix server metric labels on early close

[becomeStar]

This addresses the server-side OpenTelemetry metric labeling bug from #12117 where a generated method can be recorded as grpc.method="other" if streamClosed() happens before serverCallStarted().

What changed

• add an internal StatsTraceContext.ServerCallMethodListener hook so tracers can consume an already-resolved primary-registry MethodDescriptor
• resolve the immutable internal primary registry on the transport path and seed method classification before the async MethodLookup path runs
• keep fallback registry lookup on the existing async path
• update the OpenTelemetry server tracer to use the early-resolved method classification for close metrics

Why this shape

• avoids tracer-side HandlerRegistry lookup
• uses only the immutable internal primary registry for early transport-path lookup
• keeps fallback registry lookup on the existing async path

Tests

• primary generated method: early close preserves the generated method name
• primary non-generated method: early close still records other
• fallback generated method: fallback lookup remains on the existing async path and does not introduce early transport-path classification
• tracer-level regression: serverCallMethodResolved() + streamClosed() records the generated method name without waiting for serverCallStarted()

Notes

• ServerCallMethodListener is an internal hook that carries the resolved MethodDescriptor; tracers consume the resolved result instead of performing registry lookup themselves
• ServerImpl uses InternalHandlerRegistry explicitly for the primary registry to make it clear that the early transport- path lookup is limited to the immutable internal primary registry
• this PR intentionally does not widen transport-path lookup to the fallback registry

Ref #12117