semconv: convert process metrics to Observable instrument types

[abhay1999]

Fixes #7342

Background

Process-level metrics (memory usage, thread count, disk I/O, etc.) are
naturally read from the OS via callbacks rather than incremented inline
at a call site. Using synchronous instrument types (Int64Counter,
Int64UpDownCounter, Int64Gauge) for these makes it awkward for users
who want to register them with meter.RegisterCallback.

process.cpu.time was already using Float64ObservableCounter as the
model — this change brings the rest of the process metrics in line.

Changes

Template (semconv/templates/registry/go/weaver.yaml):
Added instrument overrides for all process metrics so future
semconv-generate runs automatically produce Observable variants.

Latest generated output (semconv/v1.40.0/processconv/metric.go):
Converted the following instruments to their Observable counterparts:

Metric	Before	After
process.context_switches	Int64Counter	Int64ObservableCounter
process.cpu.utilization	Int64Gauge	Int64ObservableGauge
process.disk.io	Int64Counter	Int64ObservableCounter
process.memory.usage	Int64UpDownCounter	Int64ObservableUpDownCounter
process.memory.virtual	Int64UpDownCounter	Int64ObservableUpDownCounter
process.network.io	Int64Counter	Int64ObservableCounter
process.paging.faults	Int64Counter	Int64ObservableCounter
process.thread.count	Int64UpDownCounter	Int64ObservableUpDownCounter
process.unix.file_descriptor.count	Int64UpDownCounter	Int64ObservableUpDownCounter
process.uptime	Float64Gauge	Float64ObservableGauge
process.windows.handle.count	Int64UpDownCounter	Int64ObservableUpDownCounter

The Add/AddSet/Record/RecordSet convenience methods are removed
from these types since Observable instruments use registered callbacks
instead of synchronous recording. The "context" and "sync" imports
and the addOptPool/recOptPool variables are also removed as they are
no longer needed.

Testing

Updated package compiles cleanly:

go build ./semconv/v1.40.0/processconv/...

[dashpole]

Semconv is a stable artifact. We can't regenerate existing versions that have already been released. You can make the change to the template, and paste what it would do to the latest version in the description (so we know it works), but we can't update older semconv directories.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.6%. Comparing base (036414b) to head (bed5db7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8235   +/-   ##
=====================================
  Coverage   82.6%   82.6%           
=====================================
  Files        310     310           
  Lines      24647   24647           
=====================================
+ Hits       20370   20375    +5     
+ Misses      3897    3893    -4     
+ Partials     380     379    -1     

see 2 files with indirect coverage changes

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[abhay1999]

Thanks for the clarification! Makes sense — I've updated the PR to only touch the template (weaver.yaml) and the latest unreleased v1.40.0 output. Dropped all the changes to the released versions.

[dmathieu]

1.40.0 has already been released. https://pkg.go.dev/go.opentelemetry.io/otel@v1.43.0/semconv/v1.40.0