refactor: PushMetricExporter hide implementation for ResourceMetrics

[fraillt]

Changes

Changed PushMetricExporer::export signature. fn export(&self, metrics: ResourceMetricsRef<'_>) -> .... It is very similar to LogExporter::export, with one differences, it doesn't expose iter() method on items (metrics), but provide a collect function which accepts a lambda function which receives a lending iterator as an argument.

This design, although doesn't look very friendly from user perspective, allow to maximum flexibility on internal implementation side.

• collect function limits lock scope, while we iterate over internal implementation.
• lending iterator allows zero-allocation collection implementation (PoC refactor: PushMetricExporter interface #2921).

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)