opentelemetry-exporter-otlp-proto-http: fix metric self-observability over-count on batch split (#5370)

* opentelemetry-exporter-otlp-proto-http: fix metric self-observability over-count on batch split

When max_export_batch_size splits an export, each split was recording the
whole batch's data-point count in the exporter's self-observability metrics
(otel.sdk.exporter.metric_data_point.*), over-counting exported/inflight data
points. Each split now records its own data-point count.

* opentelemetry-exporter-otlp-proto-http: dedupe metric data-point counting

Use the single _count_data_points helper for both the unsplit request and each
split batch, removing the duplicate count loop in export().

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Co-authored-by: Lukas Hering <40302054+herin049@users.noreply.github.com>
Co-authored-by: Emídio <9735060+emdneto@users.noreply.github.com>