collector: pass cpu and textfile config through runtime

[nicolastakashi]

Summary

This continues the incremental refactor to make node_exporter easier to embed outside the standalone binary.

The broader goal is to support ongoing work to make Prometheus exporters embeddable inside the Prometheus OpenTelemetry Collector distribution as native receivers, without requiring each exporter to be rewritten as an OpenTelemetry component.

This PR builds on the earlier config, runtime, binary wiring, and runtime-state groundwork by moving a first slice of collector-specific configuration behind runtime-owned state.

Specifically, this PR:

• snapshots shared collector path settings into runtime-owned config
• snapshots cpu collector settings into runtime-owned config
• snapshots textfile collector settings into runtime-owned config
• configures the cpu collector from runtime state instead of reading package-global flags during collector setup and update
• configures the textfile collector from runtime state instead of reading package-global flags during collector setup
• adds tests covering runtime-local config snapshot behavior for both collectors

Why

The goal of this change is to start moving individual collectors off mutable package-global configuration and onto explicit runtime-owned inputs.

Before this change, the cpu and textfile collectors read their configuration directly from package-global CLI flag state. That made runtime behavior depend on mutable process-global values and limited how cleanly a runtime instance could be constructed and reused as an embeddable component.
After this change, collector.Runtime snapshots the relevant collector configuration once and applies it when constructing/configuring collectors. Derived filtered runtimes continue to use the base runtime's captured collector configuration instead of re-reading current global flag values.
This is intentionally a narrow first collector slice rather than a broad collector-wide rewrite. The intent of this PR is to establish and validate the pattern for moving collector-specific configuration behind runtime-owned state while preserving the current standalone binary behavior.

Stack

This PR is part of a stacked series and should be reviewed after the earlier config, runtime, binary wiring, and collector runtime-state groundwork PRs.

• depends on config: add reusable exporter config package #3660
• depends on collector: add reusable runtime wrapper #3661
• depends on main: wire node_exporter through reusable config and runtime #3663
• depends on collector: scope collector runtime state #3664

Those earlier PRs introduced reusable config and runtime layers and reduced shared runtime mutation. This PR starts applying that foundation to concrete collector configuration.

[nicolastakashi]

Relates to prometheus/opentelemetry-collector-bridge#41