feat: Add out-of-band node metrics

[bgins]

Description

This PR adds the following features:

• Add system metrics
• Add process metrics
• Add network metrics
• Add load average metric
• Add database size metric
• Add OS and kernel log entry
• Add Prometheus exporter with endpoint for scraping
• Move integration test utils into a separate module
• Break Homestar process kills into *nix and windows functions
• Switch from serial to file_serial in integration tests
• Add taplo to pre-commit config and nix flake

Link to issue

Implements #184

Type of change

• New feature (non-breaking change that adds functionality)
• Refactor

Test plan (required)

We have an integration test in homestar-runtime/tests/metrics.rs.

[codecov]

Codecov Report

Merging #299 (7a023c2) into main (93a11c4) will decrease coverage by 0.06%.
The diff coverage is 74.34%.

@@            Coverage Diff             @@
##             main     #299      +/-   ##
==========================================
- Coverage   72.87%   72.81%   -0.06%     
==========================================
  Files          67       70       +3     
  Lines        6706     6935     +229     
==========================================
+ Hits         4887     5050     +163     
- Misses       1819     1885      +66     

Files Changed	Coverage Δ
homestar-runtime/src/db.rs	76.30% <ø> (ø)
homestar-runtime/src/metrics/node.rs	69.58% <69.58%> (ø)
homestar-runtime/src/metrics.rs	100.00% <100.00%> (ø)
homestar-runtime/src/metrics/exporter.rs	100.00% <100.00%> (ø)
homestar-runtime/src/runner.rs	70.87% <100.00%> (+0.18%)	⬆️
homestar-runtime/src/settings.rs	96.38% <100.00%> (+0.02%)	⬆️

... and 3 files with indirect coverage changes

[zeeshanlakhani]

Updated #184 to capture this as one of the 3 main tiers of metrics.

[zeeshanlakhani]

@bgins few minor things, but this is looking great.

[zeeshanlakhani]

great work @bgins