Add /metrics Prometheus endpoint to daemon mode

Exposes a gauge per host tracking the Unix timestamp of the last records
file update. Excluded hosts (from the excluded_host table) are labeled
excluded="true" so alerting rules can filter them out. Uses manual
Prometheus text format to avoid adding a new dependency.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 128 insertions, 0 deletions

diff --git a/internal/daemon/metrics_test.go b/internal/daemon/metrics_test.go
new file mode 100644
index 0000000..536f72c
--- /dev/null
+++ b/internal/daemon/metrics_test.go
@@ -0,0 +1,128 @@
+package daemon
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"codeberg.org/snonux/goprecords/internal/storage"
+)
+
+func TestMetricsEndpointEmpty(t *testing.T) {
+	statsDir := t.TempDir()
+	srv := httptest.NewServer(testHandler(t, statsDir))
+	defer srv.Close()
+	res, err := http.Get(srv.URL + "/metrics")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		t.Fatalf("status %d", res.StatusCode)
+	}
+	ct := res.Header.Get("Content-Type")
+	if !strings.HasPrefix(ct, "text/plain") {
+		t.Fatalf("Content-Type %q", ct)
+	}
+	body, _ := io.ReadAll(res.Body)
+	if !strings.Contains(string(body), "goprecords_host_records_last_update_timestamp_seconds") {
+		t.Fatalf("missing metric name in body: %q", body)
+	}
+}
+
+func TestMetricsEndpointWithHost(t *testing.T) {
+	statsDir := t.TempDir()
+	recordsFile := filepath.Join(statsDir, "myhost.records")
+	if err := os.WriteFile(recordsFile, []byte("1:1:Linux 5.0\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	srv := httptest.NewServer(testHandler(t, statsDir))
+	defer srv.Close()
+	res, err := http.Get(srv.URL + "/metrics")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		t.Fatalf("status %d", res.StatusCode)
+	}
+	body, _ := io.ReadAll(res.Body)
+	s := string(body)
+	if !strings.Contains(s, `host="myhost"`) {
+		t.Fatalf("missing host label: %q", s)
+	}
+	if !strings.Contains(s, `excluded="false"`) {
+		t.Fatalf("missing excluded=false label: %q", s)
+	}
+}
+
+func TestMetricsEndpointWithExcludedHost(t *testing.T) {
+	statsDir := t.TempDir()
+	dbPath := filepath.Join(t.TempDir(), "test.db")
+	recordsFile := filepath.Join(statsDir, "exchost.records")
+	if err := os.WriteFile(recordsFile, []byte("1:1:Linux 5.0\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	ctx := context.Background()
+	db, err := storage.Open(ctx, dbPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+	if err := storage.CreateSchema(ctx, db); err != nil {
+		t.Fatal(err)
+	}
+	if err := storage.AddExcludedHost(ctx, db, "exchost", "test"); err != nil {
+		t.Fatal(err)
+	}
+	db.Close()
+	h := metricsHandler(statsDir, dbPath)
+	req := httptest.NewRequest(http.MethodGet, "/metrics", nil)
+	w := httptest.NewRecorder()
+	h(w, req)
+	if w.Code != http.StatusOK {
+		t.Fatalf("status %d", w.Code)
+	}
+	body := w.Body.String()
+	if !strings.Contains(body, `excluded="true"`) {
+		t.Fatalf("expected excluded=true for exchost: %q", body)
+	}
+}
+
+func TestMetricsMethodNotAllowed(t *testing.T) {
+	statsDir := t.TempDir()
+	srv := httptest.NewServer(testHandler(t, statsDir))
+	defer srv.Close()
+	req, _ := http.NewRequest(http.MethodPost, srv.URL+"/metrics", nil)
+	res, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	res.Body.Close()
+	if res.StatusCode != http.StatusMethodNotAllowed {
+		t.Fatalf("status %d want 405", res.StatusCode)
+	}
+}
+
+func TestBuildMetricsNoDBPath(t *testing.T) {
+	statsDir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(statsDir, "h1.records"), []byte("1:1:Linux 5.0\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	body, err := buildMetrics(context.Background(), statsDir, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	s := string(body)
+	if !strings.Contains(s, `host="h1"`) {
+		t.Fatalf("missing h1 in output: %q", s)
+	}
+	if !strings.Contains(s, `excluded="false"`) {
+		t.Fatalf("missing excluded=false: %q", s)
+	}
+}