OTLP: return correct response type for partial successes

[felixbarny]

Now returns an ExportMetricsServiceResponse with an embedded ExportMetricsPartialSuccess rather than a plain ExportMetricsServiceResponse.

This also limits the size of the error message to avoid decoding issues in the collector. For indexing errors, it returns one sample error message per unique index and status code. It also limits the number of ignored data point messages to 10.

Example error message from the collector:

2025-11-07T11:54:18.241+0100    warn    otlphttpexporter@v0.134.0/otlp.go:391   Partial success response        {"resource": {"service.instance.id": "290692f3-8eed-41e9-b4f6-039bd39f9a7b", "service.name": "otelcol", "service.version": ""}, "otelcol.component.id": "otlphttp/elasticsearch-sending-queue-batch", "otelcol.component.kind": "exporter", "otelcol.signal": "metrics", "message": "Index [.ds-metrics-hostmetricsreceiver.otel-default-2025.11.07-000001] returned status [CONFLICT] for 512 documents. Sample error message: [.ds-metrics-hostmetricsreceiver.otel-default-2025.11.07-000001/AbkfLh8NQX6alMdlz69qlA][[.ds-metrics-hostmetricsreceiver.otel-default-2025.11.07-000001][0]] org.elasticsearch.index.engine.VersionConflictEngineException: [jXGZEw7bHkoQb-kpAAABml35mLA][kKxc2UpACbqPQ3lcLUGVmSkWgVcT@2025-11-07T11:00:30.000Z]: version conflict, document already exists (current version [1])\n", "dropped_data_points": 512}

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[elasticsearchmachine]

Hi @felixbarny, I've created a changelog YAML for you.