Add ast-metrics to Code Analysis

[Halleck45]

Required links

• Forge link (github.com, gitlab.com, etc): https://github.com/ast-metrics/ast-metrics
• pkg.go.dev: https://pkg.go.dev/github.com/ast-metrics/ast-metrics
• goreportcard.com: https://goreportcard.com/report/github.com/ast-metrics/ast-metrics
• Coverage service link: https://app.codecov.io/gh/ast-metrics/ast-metrics

Pre-submission checklist

• I have read the Contribution Guidelines
• I have read the Quality Standards

Repository requirements

• The repo has a go.mod file and at least one SemVer release (vX.Y.Z). Latest is v0.40.2.
• The repo has an open source license (MIT).
• The repo documentation has a pkg.go.dev link.
• The repo documentation has a goreportcard link.
• The repo documentation has a coverage service link.
• The repo has a continuous integration process (GitHub Actions).
• CI runs tests that must pass before merging.

Pull Request content

• This PR adds/removes/changes only one package.
• The package has been added in alphabetical order (between apicompat and asty).
• The link text is the exact project name.
• The description is clear, concise, non-promotional, and ends with a period.
• The link in README.md matches the forge link above.

Category quality

• The packages around my addition still meet the Quality Standards.

I checked the five entries surrounding my addition (apicompat, asty,
blanket, ChainJacking, Chronos). None of them is archived and all links
resolve, so I did not remove anything. For the record, three of them look
dormant: apicompat (last push 2017), Chronos (2022) and asty (2023). I
left them in place to keep this PR to a single item, but I am happy to open
separate removal PRs if maintainers want.

About the project

AST Metrics is a static code analyzer written in Go. It parses source code with
tree-sitter and computes maintainability metrics (cyclomatic and cognitive
complexity, Halstead volume, afferent/efferent coupling, LCOM, maintainability
index, community detection on the dependency graph), then reports them as HTML,
JSON, Markdown, SARIF or OpenMetrics. It analyzes Go alongside PHP, Python, Rust
and others, ships as a single binary with no server or runtime dependency, and
is MIT licensed. First commit is from 2024, and it is under active development.

[github-actions]

Automated Quality Checks

Required checks

✅ Repo: accessible, has go.mod and SemVer release
✅ pkg.go.dev: OK
✅ Go Report Card: OK (grade unknown)

Additional checks

✅ License: MIT
✅ Maturity: repo has 5+ months of history
✅ CI/CD: GitHub Actions workflows detected
✅ README: present
✅ Coverage: link accessible

These checks are automated and do not replace maintainer review. See CONTRIBUTING.md for full guidelines.

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PR Diff Validation

Content checks

✅ Files changed: only README.md
✅ Single item: one package added
✅ Link consistency: README link matches forge link in PR body
✅ Link text: matches repository name
✅ Description: ends with punctuation
✅ Description tone: no promotional language detected
✅ Category size: Code Analysis has 33 items

Automated diff validation — does not replace maintainer review.