refactor: refactor rs/ic_os/metrics: shared write helper, replace hand-rolled formatter, rename tools

[shilingwang]

#NODE-1524

Summary

This PR cleans up the five metric-writing tools under rs/ic_os/ in three ways:

1. Shared atomic write helper (rs/ic_os/metrics/utils)
   A new ic-os-metrics-utils crate provides a single write_registry_to_file(registry, path) function. It encodes a Prometheus registry and writes the result atomically via a temp-file rename (using ic_sys::write_string_using_tmp_file), so a partially-written .prom file is never visible to node_exporter.
   Previously, metrics_tool, nft_exporter, and guest_vm_runner all duplicated the same 6-line BufWriter / TextEncoder / flush block and wrote non-atomically. All four are now updated to use the shared helper.
2. Replace hostos_tool's hand-rolled Prometheus formatter
   hostos_tool/src/prometheus_metric.rs (158 lines) reimplemented the Prometheus text exposition format from scratch, used f32 for metric values, and only supported writing a single metric. It is deleted and replaced with the standard prometheus crate (GaugeVec with a gen label), consistent with the rest of the codebase.
3. Rename tools for consistency
   The _tool suffix was used inconsistently — hostos_tool, guestos_tool, setupos_tool are all configuration/provisioning binaries, while fstrim_tool and metrics_tool are periodic metric exporters that live in rs/ic_os/metrics/. Both are renamed to match the folder's naming convention:

• metrics_tool → custom_metrics (binary, crate, systemd units)
• fstrim_tool → fstrim_metrics (binary, crate, library, systemd units)

The public entry-point function fstrim_tool(...) in the library crate is also simplified to run(...).
4. Fix integration test metric ordering
fstrim_metrics integration tests were asserting on a hardcoded metric ordering that did not match the alphabetical order Prometheus registries actually produce (fstrim_datadir_* < fstrim_last_* < fstrim_runs_*). All three affected expected strings are corrected.

Files changed

Area	Change
rs/ic_os/metrics/utils/	New shared crate
rs/ic_os/metrics/custom_metrics/	Renamed from metrics_tool; uses shared helper
rs/ic_os/metrics/fstrim_metrics/	Renamed from fstrim_tool; integration test assertions fixed
rs/ic_os/metrics/nft_exporter/	Uses shared helper (atomic writes)
rs/ic_os/os_tools/guest_vm_runner/src/metrics.rs	Uses shared helper (atomic writes)
rs/ic_os/os_tools/hostos_tool/	Deletes prometheus_metric.rs; uses prometheus crate + shared helper
ic-os/components/monitoring/guestos/	Renamed metrics_tool.* and fstrim_tool.* systemd units
rs/tests/crypto/	Updated for fstrim_metrics rename and new crate name

[andrewbattat]

Nice!