cli: add --json output mode for scripting

[Hollujay]

Summary

fetch-metrics prints a human-readable block of parsed attestation values.
That is right for a person at a terminal, but it means anyone wiring this into
a cron job or CI has to parse formatted text.

Acceptance Criteria

• Add a --json flag that emits the same parsed values as a single JSON
  object on stdout
• Keep the existing human-readable output as the default — do not change
  current behaviour
• Ensure nothing else (logs, warnings) is written to stdout in --json
  mode, so output stays pipeable
• Test covering both output modes

Tech Stack

Rust, serde_json