Add observability metrics for lockfile refresh and validation operations

[coderabbitai]

Background

Raised during review of PR #75 (refresh stale Cargo lockfiles before publish preflight).

Problem

lading/commands/lockfile.py uses LOGGER calls to record decisions at key points, but provides no structured metrics (counters, histograms, timers) for:

• Lockfile discovery count (number of tracked lockfiles found)
• Lockfile refresh duration and error rate
• Lockfile freshness validation latency and failure rate

Without these, operators cannot diagnose throughput problems, track error trends, or alert on regressions in lockfile handling.

Suggested resolution

• Choose and integrate a lightweight Python metrics framework (e.g., prometheus_client or a project-standard library) at the lading/commands/lockfile.py boundary.
• Emit a counter on each successful and failed refresh_lockfile invocation.
• Emit a histogram or timer around refresh_lockfile and validate_lockfile_freshness call duration.
• Emit a gauge or counter for the number of lockfiles discovered by discover_tracked_lockfiles.
• Document the metric names and labels in docs/developers-guide.md.

References

• PR: Refresh stale Cargo lockfiles before publish preflight (#61) #75
• Requested by: @leynos

[coderabbitai]

🔗 Related PRs

leynos/episodic#98 - Document checkpoint atomicity boundaries (2.4.2) (#91) [merged]
leynos/repovec-appliance#37 - Serialize qdrant key provisioning (#29) [merged]
leynos/ortho-config#334 - Document observability and dependency management practices [merged]

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