Comments about the semantic conventions for hardware metrics

[sebastien-rosset]

In https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/semantic_conventions/hardware-metrics.md#hw---common-hardware-metrics

Serial Number attribute

There seems to be some inconsistencies for the listed attributes.

Many of the listed components (battery, disk controller, GPU, fans, memory, etc) can have a serial number. It's true a low-end battery may not have a serial number, but more advanced batteries can have a serial number and some sensors such as temperature. It's also true "temperature" itself does not have a serial number, but for example an enclosure may have inlet and outlet temperature sensors.

[sebastien-rosset]

Opened #3137, #3138, #3139 and #3140