feat: add pressure stall information (PSI) to host instrumentation

[alpineQ]

Description

This PR adds support for Linux Pressure Stall Information (PSI) metrics to the instrumentation/host package. PSI metrics provide valuable insights into resource pressure on CPU, memory, and I/O subsystems, available on Linux systems with kernel 4.20+.

The implementation adds 20 new metric instruments:

• CPU: system.psi.cpu.some.* (avg10, avg60, avg300, total)
• Memory: system.psi.memory.some.* and system.psi.memory.full.* (avg10, avg60, avg300, total for each)
• I/O: system.psi.io.some.* and system.psi.io.full.* (avg10, avg60, avg300, total for each)

Where "some" indicates some tasks are stalled and "full" indicates all tasks are stalled. The avg* metrics represent pressure averages over 10, 60, and 300 second windows, while total metrics track cumulative stall time in microseconds.

PSI metrics are automatically collected alongside existing host metrics when running on Linux systems with PSI support. On non-Linux systems or when PSI is unavailable, the implementation gracefully degrades with no impact.

Link to tracking issue

Closes #8082

[alpineQ]

I understand that metric definitions should ideally be managed through the weaver tool and its YAML schema files. However, I noticed that the weaver configuration files are maintained in a separate repository, and I wasn't able to find documentation on the coordinated update process for both the schema definitions and the Go instrumentation code.

I'm happy to submit corresponding changes to the weaver repository if needed. If this is the preferred approach, I would appreciate guidance on:

• The specific repository and file locations for PSI metric definitions
• The recommended workflow for coordinating changes across both repositories