cli: support additional metrics exposition formats

[Hollujay]

Summary

fetch-metrics parses one metrics format today. Operators run different
stacks, and a provider whose node exposes metrics in another shape currently
has no path to using this tool without hand-computing values.

Acceptance Criteria

• Identify which additional format(s) are actually worth supporting —
  propose in this thread before implementing
• Implement parsing for the agreed format behind explicit detection or a
  flag, not silent guessing
• Add a sample file under examples/ for each newly supported format
• Tests parsing each sample to expected uptime/latency values

Tech Stack

Rust