Prometheus exporter should use NaN instead of Nan

### What happened?

Minor detail I found while reading the prometheus exporter:

It [currently emits](https://github.com/open-telemetry/opentelemetry-js/blob/b152497e5f77f35c5e49606da5b894bb8e5af00f/experimental/packages/opentelemetry-exporter-prometheus/src/PrometheusSerializer.ts#L105) the string "Nan" when recording not a number values.

According to the [Prometheus](https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md#comments-help-text-and-type-information) and [OpenMetrics](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#values) specs, this should be the string: "NaN":.

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Prometheus exporter should use NaN instead of Nan #3465

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