Preserve metric names during metricrequest

[lsj4401]

Description

This PR fixes an issue in exporter/exporterhelper's sending_queue where metric names were lost during metricRequest splitting. Previously, when large batches of metrics were processed, the splitting logic in metric_batch.go could cause the name field of some metrics to disappear. This fix ensures that all metric fields are properly preserved when metricRequest objects are split.

Link to tracking issue

Fixes #13236

[linux-foundation-easycla]

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