Client logging and metrics

[piceri]

This change:

• Changes not_saved metric to no_attestation
• Adds rate_limited metric
• Document new metrics in README
• Refactor client logging to better handle rate limiting and 404s for no artifact found
• Add tests for PostOne

[Copilot]

Pull request overview

This PR updates the deployment record client’s observability and behavior around GitHub API responses (rate limiting and “no artifacts found”), including new Prometheus metrics and expanded PostOne test coverage.

Changes:

• Add new Prometheus counters for no_attestation and rate_limited outcomes.
• Refactor PostOne response handling/logging to distinguish rate limiting and “no artifacts found” 404s.
• Add comprehensive PostOne unit tests and document the new metrics in the README.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
pkg/dtmetrics/prom.go	Adds new counters for no_attestation and rate_limited.
pkg/deploymentrecord/client.go	Introduces NoArtifactError and refines retry/logging logic for 4xx/5xx, including rate-limited detection.
pkg/deploymentrecord/client_test.go	Adds table-driven tests validating PostOne behavior and metric deltas across outcomes.
internal/controller/controller.go	Treats NoArtifactError as non-fatal (no requeue) when posting records.
README.md	Documents the newly added metrics.
go.mod	Adds an indirect dependency entry.

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