feat(otlptrace): add SyncClient and SyncExporter with arena reuse

[marwan562]

Fixes #8860

Problem

The current Exporter/Client contract does not guarantee that telemetry passed to Client.UploadTraces is fully consumed when the method returns. A client may retain or process it asynchronously, which prevents safe reuse of arena-backed protobuf memory. PR #8425 demonstrated the allocation/CPU opportunity but reuse was incompatible with the existing contract.

Solution

Introduce a synchronous ownership boundary:

• SyncClient in go.opentelemetry.io/otel/exporters/otlp/otlptrace extends Client with UploadTracesSync. Contract: MUST fully consume protoSpans before returning and MUST NOT retain/reference/mutate after return.
• SyncExporter (NewSync/NewSyncUnstarted) uses UploadTracesSync exclusively and reuses an arena via a bounded sync.Pool. Arena is internal; no arena-backed object escapes the synchronous operation.
• Options (WithInitialBatchSize, WithMaxRetainedBatchSize) allow tuning of preallocation and bounded retention (large arenas are discarded).
• internal/tracetransform adds Arena with chunkedStorage for KeyValue/AnyValue plus SpansWithArena/KeyValuesWithArena etc. Existing Spans path remains heap-allocated for backward compatibility.
• HTTP and gRPC clients implement SyncClient (both now assert var _ SyncClient = (*client)(nil)) by factoring UploadTraces into uploadTraces helper so both sync and async paths share the synchronous transport logic (eager marshal for HTTP, retained pbRequest only for the duration of requestFunc for gRPC).

Arena details

• NewArena(size) preallocates size*8 KeyValue/AnyValue slots plus slices for string/bool/int/double wrappers.
• Reset clears with clear() and Reset() on protobuf messages; Exceeds(maxSpans) implements bounded retention.
• SyncExporter.ExportSpans gets arena from pool, calls SpansWithArena, invokes UploadTracesSync, then resets and conditionally returns to pool.

Testing

• Added sync_test.go for SyncExporter: sync dispatch, error wrapping, MarshalLog filtering, arena reuse, and Option handling.
• Added arena_sync_test.go verifying SpansWithArena matches heap Spans for attributes/events/links, arena reuse, and Exceeds.
• Existing tests pass: go test ./exporters/otlp/otlptrace/... and .../otlptracehttp and .../otlptracegrpc (including race).

Prior Art

Follows the proposal in #8860 and reuses logic from #8425 but confines pooling to SyncExporter where the contract makes it safe. Existing Client semantics are unchanged.

Closes #8860

[linux-foundation-easycla]

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