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, 100 insertions, 0 deletions

diff --git a/internal/daemon/metrics.go b/internal/daemon/metrics.go
new file mode 100644
index 0000000..192db77
--- /dev/null
+++ b/internal/daemon/metrics.go
@@ -0,0 +1,100 @@
+package daemon
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"codeberg.org/snonux/goprecords/internal/recordsdir"
+	"codeberg.org/snonux/goprecords/internal/storage"
+)
+
+func metricsHandler(statsDir, dbPath string) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodGet {
+			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+			return
+		}
+		body, err := buildMetrics(r.Context(), statsDir, dbPath)
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+		w.Header().Set("Content-Type", "text/plain; version=0.0.4; charset=utf-8")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write(body)
+	}
+}
+
+func buildMetrics(ctx context.Context, statsDir, dbPath string) ([]byte, error) {
+	entries, err := recordsdir.ListNonEmptyFiles(statsDir)
+	if err != nil {
+		return nil, fmt.Errorf("list records: %w", err)
+	}
+	excluded, err := loadExcludedSet(ctx, dbPath)
+	if err != nil {
+		return nil, fmt.Errorf("load excluded hosts: %w", err)
+	}
+	var sb strings.Builder
+	writeMetricHeader(&sb)
+	for _, e := range entries {
+		mtime, err := recordsMtime(statsDir, e.Host)
+		if err != nil {
+			continue
+		}
+		isExcluded := excluded[e.Host]
+		writeHostMetric(&sb, e.Host, mtime, isExcluded)
+	}
+	return []byte(sb.String()), nil
+}
+
+func writeMetricHeader(sb *strings.Builder) {
+	sb.WriteString("# HELP goprecords_host_records_last_update_timestamp_seconds Unix timestamp of the last records file update for each host.\n")
+	sb.WriteString("# TYPE goprecords_host_records_last_update_timestamp_seconds gauge\n")
+}
+
+func writeHostMetric(sb *strings.Builder, host string, mtime int64, excluded bool) {
+	excludedVal := "false"
+	if excluded {
+		excludedVal = "true"
+	}
+	fmt.Fprintf(sb, "goprecords_host_records_last_update_timestamp_seconds{host=%q,excluded=%q} %d\n",
+		host, excludedVal, mtime)
+}
+
+func recordsMtime(statsDir, host string) (int64, error) {
+	path := filepath.Join(statsDir, host+".records")
+	fi, err := os.Stat(path)
+	if err != nil {
+		return 0, err
+	}
+	return fi.ModTime().Unix(), nil
+}
+
+func loadExcludedSet(ctx context.Context, dbPath string) (map[string]bool, error) {
+	if dbPath == "" {
+		return map[string]bool{}, nil
+	}
+	db, err := storage.Open(ctx, dbPath)
+	if err != nil {
+		return map[string]bool{}, nil
+	}
+	defer db.Close()
+	return loadExcludedHosts(ctx, db)
+}
+
+func loadExcludedHosts(ctx context.Context, db *sql.DB) (map[string]bool, error) {
+	hosts, err := storage.LoadExcludedHosts(ctx, db)
+	if err != nil {
+		return nil, err
+	}
+	set := make(map[string]bool, len(hosts))
+	for _, h := range hosts {
+		set[h.Host] = true
+	}
+	return set, nil
+}