Complete process metrics for CPU

[mariomac]

This PR provides the process.cpu.time and process.cpu.utilisation metrics according to the definition here: https://opentelemetry.io/docs/specs/semconv/system/process-metrics/

It also fixes the way the process.cpu.utilisation value was previously provided.

New metrics are expirable and their attributes are selectable. This added extra complexity:

• If the user keeps the process.cpu.state attribute enabled, the CPU metrics need to be explicitly separated by user/system/wait time. This attribute needs to be explicitly added from the exporters, as it can't be inferred from the process.Status record.
• If the user disables the process.cpu.state attribute, the CPU metrics need to be added into a single metric.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.95604% with 11 lines in your changes missing coverage. Please review.

Project coverage is 79.05%. Comparing base (26dc529) to head (db6ccb2).

Files	Patch %	Lines
pkg/internal/infraolly/process/snapshot.go	80.00%	3 Missing and 3 partials ⚠️
pkg/internal/export/otel/metrics_proc.go	93.87%	2 Missing and 1 partial ⚠️
pkg/internal/export/otel/common.go	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #911      +/-   ##
==========================================
+ Coverage   73.66%   79.05%   +5.39%     
==========================================
  Files         131      131              
  Lines        9755     9832      +77     
==========================================
+ Hits         7186     7773     +587     
+ Misses       2093     1556     -537     
- Partials      476      503      +27     

Flag	Coverage Δ
integration-test	55.16% <73.07%> (+0.03%)	⬆️
k8s-integration-test	58.62% <50.00%> (?)
oats-test	35.69% <2.74%> (-0.26%)	⬇️
unittests	45.02% <91.75%> (+1.56%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[grcevski]

LGTM! Very cool!