Fix EMF exporter to generate EMF metrics when dimensions are empty due to no attributes

[jj22ee]

Description of changes:
Populate the correct CloudWatchMetrics in EMF when metrics has empty dimension list due to no attributes.

Testing:

{
    "_aws": {
        "Timestamp": 1753213216039,
        "CloudWatchMetrics": [
            {
                "Namespace": "default",
                "Metrics": [
                    {
                        "Name": "histogram.counter",
                        "Unit": "Milliseconds"
                    }
                ]
            }
        ]
    },
    "Version": "1",
    "otel.resource.service.name": "unknown_service:node",
    "otel.resource.telemetry.sdk.language": "nodejs",
    "otel.resource.telemetry.sdk.name": "opentelemetry",
    "otel.resource.telemetry.sdk.version": "1.30.1",
    "otel.resource.telemetry.auto.version": "0.6.0-dev0-aws",
    "otel.resource.aws.service.type": "gen_ai_agent",
    "histogram.counter": {
        "Values": [
            0.9946140065969877,
            1.9892280131939772,
            3.002072155884692,
            3.978456026387959,
            0
        ],
        "Counts": [
            1,
            1,
            2,
            3,
            2
        ],
        "Count": 9,
        "Sum": 21,
        "Max": 4,
        "Min": 0
    }
}

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