feat(metrics): opt-in Prometheus /metrics on the loopback fleet server

[GeiserX]

What

Adds an opt-in Prometheus /metrics endpoint on the existing loopback fleet server, so a homelab Prometheus/Grafana can scrape the app. The production CashPilot has this; the rewrite didn't. (Gap analysis.)

• Gauges (cashpilot_ prefix, mirroring the original): cashpilot_up, cashpilot_collect_interval_minutes, cashpilot_earnings_usd_total/_today/_month, cashpilot_service_balance{platform,currency} (+ cashpilot_service_error{platform} for a latest-error platform, no bogus balance), cashpilot_service_health_score{slug}, cashpilot_service_uptime_percent{slug}, cashpilot_fleet_devices{status}, cashpilot_fleet_device_last_seen_seconds{device}.
• No new dependency — hand-rolled text exposition (escaped labels, text/plain; version=0.0.4).
• Reuses computeEarningsSummary — the FX/accrual logic is not duplicated.
• Opt-in + gated: new MetricsEnabled config flag (default false); the route is only registered when enabled → 404 otherwise. Unauthenticated by Prometheus convention, on the loopback-default bind.

Tests

Handler output (metric lines, content-type, error-row handling, label escaping, nil-store safety, non-GET rejection) + the mux gating (404 disabled / 200 enabled) + config default. Verified on macOS: go build/vet/test -race ./... all green.

Summary by CodeRabbit

• New Features

  • Added an optional Prometheus-compatible /metrics endpoint on the fleet server.
  • Included live status, collection interval, earnings, balance, service health, uptime, fleet device counts, and device staleness metrics.
  • Kept core fleet health routes available, with metrics exposure controlled by configuration.

• Bug Fixes

  • Improved metrics output handling for non-GET requests and missing data.
  • Ensured metric labels are safely escaped and output stays consistent.

[codecov]

Codecov Report

❌ Patch coverage is 94.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.38%. Comparing base (1d48731) to head (bcfcad1).

Files with missing lines	Patch %	Lines
metrics.go	93.47%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
+ Coverage   65.05%   66.38%   +1.32%     
==========================================
  Files           8        9       +1     
  Lines        2012     2109      +97     
==========================================
+ Hits         1309     1400      +91     
- Misses        611      614       +3     
- Partials       92       95       +3     

Files with missing lines	Coverage Δ
fleet_server.go	78.57% <100.00%> (+0.88%)	⬆️
internal/config/config.go	67.59% <ø> (ø)
metrics.go	93.47% <93.47%> (ø)

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 832b4681-d344-42bd-b9f0-90bfd4e2d6ab

📥 Commits

Reviewing files that changed from the base of the PR and between 1d48731 and bcfcad1.

📒 Files selected for processing (5)

• fleet_server.go
• internal/config/config.go
• internal/config/config_test.go
• metrics.go
• metrics_test.go

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📝 Walkthrough

Walkthrough

Adds an opt-in Prometheus /metrics HTTP endpoint gated by a new MetricsEnabled config field. Fleet server route registration is refactored into a fleetMux helper. A new metrics.go renders liveness, earnings, and store-backed gauges in Prometheus text format, with accompanying tests.

Changes

Prometheus metrics endpoint

Layer / File(s)	Summary
MetricsEnabled config flag internal/config/config.go, internal/config/config_test.go	Documents the opt-in MetricsEnabled field and adds a test verifying its default false value and persistence across reload.
Fleet mux route gating fleet_server.go	Extracts route registration into a new fleetMux(metricsEnabled bool) helper; always registers health/heartbeat routes, conditionally registers /metrics.
Metrics handler and rendering metrics.go	Adds handleMetrics GET-only HTTP handler, renderMetrics/renderStoreMetrics building Prometheus gauge families (liveness, earnings, balances, error flags, health score, uptime, fleet counts, staleness), plus metricsWriter, label escaping, float formatting, and sortedKeys helpers.
Metrics endpoint tests metrics_test.go	Adds tests covering full metrics scraping/escaping, 405 on non-GET, nil-store safety, and fleet mux route gating.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Possibly related PRs

• GeiserX/CashPilot-Desktop#29: Both PRs modify fleet_server.go's startFleetAPI routing, with this PR building on prior Fleet server endpoint/routing setup.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 63.64% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: an opt-in Prometheus /metrics endpoint on the loopback fleet server.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/prometheus-metrics

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