Metrics: Added prometheus Observability metrics for syncer

[richochetclementine1315]

Closes Issue #1576

What this PR does

The syncer (backend/pkg/syncer/syncer.go) currently has no Prometheus metrics, so
there's no way to tell from the outside whether it's actually working — whether it's
successfully checking for updates, how long those checks take, or whether it's actually
creating new packages when it finds them.

This PR adds 4 new metrics and wires them into the syncer's existing code paths. No
existing behavior, function signature, or return value is changed — this is purely
additive.

Changes

backend/pkg/metrics/metrics.go

• Added 4 new metric definitions: 1 GaugeVec, 2 CounterVecs, 1 HistogramVec.
• Added 4 small exported helper functions (SyncerLastSuccessTimestamp,
  SyncerCheckFailure, SyncerCheckDuration, SyncerPackageCreated) so the syncer
  package can update these metrics without reaching into metrics-package-private
  variables.
• Registered the new collectors in registerNebraskaMetrics().
• calculateMetrics() is untouched — these new metrics are event-driven (updated live
  as the syncer runs), not pull-based like the existing DB-stats metrics, so there was
  nothing to add there.

backend/pkg/syncer/syncer.go

• Added the metrics package import.
• In checkForUpdates(): time the existing doOmahaRequest() call, record the
  duration, increment the failure counter on error, and set the last-success timestamp
  on success. The existing control flow (what returns, what loops) is unchanged.
• In createPackage(): increment the packages-created counter right before the
  existing final return pkg, nil — i.e. only after package creation has actually
  succeeded in the database.

How I tested this

1. Unit tests (isolated, no DB/network needed)

Added backend/pkg/metrics/syncer_metrics_test.go, using
github.com/prometheus/client_golang/prometheus/testutil to verify each metric updates
correctly in isolation:

$ make check

2. Live end-to-end test — success path

Ran the backend locally with the syncer enabled against the real Flatcar update
servers, then checked /metrics:

$ curl -s http://localhost:8000/metrics | grep nebraska_syncer

3. Live end-to-end test — failure path

Restarted the backend pointing -sync-update-url at an unreachable host to force a
failure:

$ go run ./cmd/nebraska -auth-mode=noop -enable-syncer \
    -sync-update-url=https://nonexistent.invalid.example/ -port=8000

4. Regression check

Confirmed the existing syncer test suite still passes unmodified:

$ export NEBRASKA_DB_URL="postgres://postgres:nebraska@127.0.0.1:5432/nebraska_tests?sslmode=disable&connect_timeout=10"
$ go test ./pkg/syncer/... -v

[Copilot]

Pull request overview

This PR adds Prometheus observability metrics for the backend syncer so operators can detect stalled/failed sync checks, monitor request latency, and confirm package creation activity.

Changes:

• Defines and registers four new syncer metrics (last success timestamp, check failures, check duration, packages created) and exposes small helper functions to update them.
• Wires metric updates into the syncer’s update-check loop and package-creation path.
• Adds unit tests validating metric updates, and updates Go module dependencies accordingly.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
backend/pkg/syncer/syncer.go	Records sync-check duration/failures/last-success and increments package-created counter.
backend/pkg/metrics/metrics.go	Adds new metric collectors, helper functions, and registers them.
backend/pkg/metrics/syncer_metrics_test.go	Adds isolated unit tests for the new syncer metrics.
backend/go.mod	Adds an indirect dependency pulled in by the new/updated test dependencies.

Suppressed comments (1)

backend/pkg/syncer/syncer.go:497

• There is a whitespace-only blank line here (visible in the diff). Please remove it (or run gofmt) so the file stays cleanly formatted.

		return nil, err
	}
    
	metrics.SyncerPackageCreated(descriptor.name, descriptor.arch.String())

---

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

Suppressed comments (4)

backend/pkg/metrics/syncer_metrics_test.go:36

• This test doesn't currently verify that an observation was recorded (only that the collector yields at least one metric). Checking the histogram's sample count/sum makes the test actually detect a broken SyncerCheckDuration implementation.

func TestSyncerCheckDuration(t *testing.T) {
	SyncerCheckDuration("alpha", "amd64", 1.5)

	count := testutil.CollectAndCount(syncerCheckDurationSeconds)
	if count == 0 {
		t.Fatal("expected histogram to have at least one observation registered")
	}
}

backend/pkg/syncer/syncer.go:229

• Line contains whitespace-only indentation (spaces on an otherwise blank line), which will fail gofmt/golines-style checks and adds noisy diffs. Make the blank line truly blank (or remove it) to keep formatting stable.

		l.Debug().Str("channel", descriptor.name).Str("arch", descriptor.arch.String()).Str("currentVersion", currentVersion).Msg("checking for updates")
        
		channel := descriptor.name
		arch := descriptor.arch.String()

backend/pkg/syncer/syncer.go:498

• There is a whitespace-only blank line before the new metrics call. This is likely to fail formatting/lint checks; replace it with a real blank line to keep gofmt output clean.

	}
    
	metrics.SyncerPackageCreated(descriptor.name, descriptor.arch.String())
	return pkg, nil

backend/pkg/metrics/syncer_metrics_test.go:7

• To validate the histogram helper, the test needs to assert the histogram's sample count/sum changes. CollectAndCount(syncerCheckDurationSeconds) can be non-zero even if the helper forgets to call Observe, because simply creating a labeled child metric makes the vec collectable.

This issue also appears on line 29 of the same file.

import (
	"testing"

	"github.com/prometheus/client_golang/prometheus/testutil"
)