[exporter/loadbalancing] retry only failed backend data

[jeevan6996]

Summary

• return consumererror partial-data errors from logs, traces, and metrics exporters when only a subset of backend exports fails
• include only failed backend payloads in retryable error data instead of returning a generic aggregated error
• add merge helpers for logs and metrics payloads to build retry subsets
• add tests covering partial-data behavior for logs and traces failures, and mixed-success metrics fanout failures

Fixes #47600.

Testing

• cd exporter/loadbalancingexporter && go test ./...

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[axw]

@jeevan6996 thanks for opening a PR. There's already support in exporterhelper for retrying partial failures -- see https://github.com/open-telemetry/opentelemetry-collector/blob/22a5e61748b9daabc6781a22e7fa97d1c2c104e9/exporter/exporterhelper/internal/request/request.go#L34-L45

Rather than introducing code specific to the loadbalancing exporter, let's see if/how that can be used.