Add OTel metric export batching support

[pkcll]

Summary

• Align OpenTelemetry metric SDK and OTLP gRPC metric exporter dependencies on v1.44.0.
• Enable the upstream experimental PeriodicReader batching path through OTEL_GO_X_METRIC_EXPORT_BATCH_SIZE.
• Add coverage for bounded batches, exactly-once data-point preservation, and invalid/unset fallback behavior.
• Document that the setting is process-wide, read during reader construction, count-based rather than byte-based, and does not replace collector receive limits.

Validation

• go test ./pkg/beholder/...
• go test ./pkg/beholder -run TestPeriodicReaderMetricExportBatchSize -count=1
• Resolved modules:
  • go.opentelemetry.io/otel/sdk/metric v1.44.0
  • go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetricgrpc v1.44.0

Notes

• No custom metric request splitter was added.
• No CRE Workflow User Metrics or CHIP Ingress behavior changed.
• Production batch-size and collector receive-limit changes remain deployment-owner follow-ups.