otel_trace_source drops the whole OTLP request when a span has duplicate attribute keys

Data Prepper 2.16.0, sink OpenSearch 3.7.0, otel_trace_source over gRPC.

When one span repeats an attribute key, the whole ExportTraceServiceRequest is dropped,
not only that span.

WARN org.opensearch.dataprepper.plugins.source.oteltrace.OTelTraceGrpcService -
Failed to parse request with error 'Duplicate key span.attributes.compute@expression
(attempted merging values esql://routine/#MyCompute.Main and
 esql://routine/#MyCompute.Main)'. Request body: resource_spans { ... }

Both merged values are the same, so even an exact duplicate is fatal.

In our traffic every request carried 8 spans, 4 with the duplicated keys and 4 valid root
spans. All 8 were lost and documentsSuccess does not move.

The producer is IBM App Connect Enterprise 13.0.8 with openTelemetryScope: extended. On
Compute node spans it emits four attributes twice, each time with identical value:

spans {
  name: "MyApp.MyFlow.logMessageEvent"
  attributes { key: "compute.expression" value { string_value: "esql://routine/#MyCompute.Main" } }
  attributes { key: "compute.mode"       value { string_value: "message" } }
  attributes { key: "nodeName"           value { string_value: "logMessageEvent" } }
  attributes { key: "nodeType"           value { string_value: "ComputeNode" } }
  attributes { key: "compute.expression" value { string_value: "esql://routine/#MyCompute.Main" } }
  attributes { key: "compute.mode"       value { string_value: "message" } }
  attributes { key: "nodeName"           value { string_value: "logMessageEvent" } }
  attributes { key: "nodeType"           value { string_value: "ComputeNode" } }
}

Pipeline is a plain one, otel_trace_source (port 21890, ssl false, unframed_requests true)
-> otel_traces -> opensearch sink with index_type: trace-analytics-raw.

Cause looks to be in OTelProtoOpensearchCodec$OTelProtoDecoder, which builds the attribute
maps with the two argument Collectors.toMap(keyMapper, valueMapper). That form has no merge
function and throws IllegalStateException on a duplicate key. OTelTraceGrpcService catches
it per request, so everything in the batch is discarded. Same pattern is in
getSpanAttributes, getResourceAttributes, getLinkAttributes and getEventAttributes,
and OTelProtoStandardCodec looks the same.

Expected: a duplicate key should not kill the request. A merge function (a, b) -> b would be
enough, it is lossless when the values are equal. If merging is not wanted, then drop only the
offending span and expose a counter for it.

I am aware the producer is not strictly spec compliant, attribute keys should be unique. But
OTLP protobuf allows a repeated KeyValue, so a receiver will meet this in the wild.
Elasticsearch 9.5.4 native OTLP endpoint accepts the same traffic with 0 errors.

Workarounds we found: set the producer to openTelemetryScope: standard, which removes the
Compute spans (verified, 0 rejected requests), or put an OpenTelemetry Collector in front only
to deduplicate the attributes.

Start in OTelProtoOpensearchCodec$OTelProtoDecoder, inspecting getSpanAttributes, getResourceAttributes, getLinkAttributes, and getEventAttributes, then compare OTelProtoStandardCodec. Run the relevant codec and otel_trace_source tests, or add focused coverage for repeated attributes. Done means a request containing duplicate keys no longer causes the whole batch to be discarded, while valid spans are processed.