feat: Populate metric unit of cpu metrics

[obs-gh-enricogiorio]

Set the unit to "CPU" when the metric name contains cpu and the unit is not already set.

[obs-gh-enricogiorio]

These are my various attempts at making the change show up in o2. All of them have failed so far.
The docs say that I can access the "unit" field from the "metric" context, but apparently it's not the case, unless I am missing something.

[obs-gh-alexlew]

for some reason even the metrics transform processor doesn't mention being able to modify a metric's unit. i wonder if its prohibited for some reason

[obs-gh-alexlew]

found this example
https://opentelemetry.io/blog/2023/any-metric-receiver/#fine-tuning-with-the-transform-processor

processors:
  transform:
    metric_statements:
      - context: datapoint
        statements:
          - set(metric.unit, attributes["unit"])
          - delete_key(attributes, "unit")

[obs-gh-enricogiorio]

Ok so this uses a lower level context. This will be more expensive for sure, I'll try it out. Thanks!

[obs-gh-enricogiorio]

I will open an issue regarding the metric context btw, because it explicitly says that you can access the unit, but it clearly doesn't work.

[obs-gh-enricogiorio]

Wait but your example also doesn't work, right? Have you tried it? I don't see any metrics showing up with the "unit" label

[obs-gh-alexlew]

I haven't tried it, just found it in the opentelemetry docs