Fix EMF exporter to generate EMF metrics with dimension empty

[mxiamxia]

Description of changes:

• Populate the correct CloudWatchMetrics in EMF when metrics has empty dimension list

Test

{
    "_aws": {
        "Timestamp": 1753139679038,
        "CloudWatchMetrics": [
            {
                "Namespace": "MyApplication1",
                "Metrics": [
                    {
                        "Name": "system_cpu_usage_percent",
                        "Unit": "Percent"
                    },
                    {
                        "Name": "http_request_duration_seconds",
                        "Unit": "Seconds"
                    }
                ]
            }
        ]
    },
    "Version": "1",
    "otel.resource.telemetry.sdk.language": "python",
    "otel.resource.telemetry.sdk.name": "opentelemetry",
    "otel.resource.telemetry.sdk.version": "1.33.1",
    "otel.resource.service.name": "my-service",
    "otel.resource.service.version": "0.1.0",
    "otel.resource.deployment.environment": "production",
    "system_cpu_usage_percent": 0.2,
    "http_request_duration_seconds": {
        "Values": [
            0.39614037455626866,
            0.4866507397084614
        ],
        "Counts": [
            1,
            1
        ],
        "Count": 2,
        "Sum": 0.8848342305738114,
        "Max": 0.4877384525234254,
        "Min": 0.397095778050386
    }
}

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