internal: initial metrics aggregator

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I9cdd6e2b33bb65182568db9db4460bc46a6a6964

internal/aggregate/metrics.go

@@ -0,0 +1,101 @@
+package aggregate
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"notashelf.dev/watchdog/internal/config"
+)
+
+// Records analytics events as Prometheus metrics
+type MetricsAggregator struct {
+	registry     *PathRegistry
+	cfg          config.Config
+	pageviews    *prometheus.CounterVec
+	customEvents *prometheus.CounterVec
+	pathOverflow prometheus.Counter
+}
+
+// Creates a new metrics aggregator with dynamic labels based on config
+func NewMetricsAggregator(registry *PathRegistry, cfg config.Config) *MetricsAggregator {
+	// Build label names based on what's enabled in config
+	labels := []string{"path"} // path is always included
+
+	if cfg.Site.Collect.Country {
+		labels = append(labels, "country")
+	}
+
+	if cfg.Site.Collect.Device {
+		labels = append(labels, "device")
+	}
+
+	if cfg.Site.Collect.Referrer != "off" {
+		labels = append(labels, "referrer")
+	}
+
+	pageviews := prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Name: "web_pageviews_total",
+			Help: "Total number of pageviews",
+		},
+		labels,
+	)
+
+	customEvents := prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Name: "web_custom_events_total",
+			Help: "Total number of custom events",
+		},
+		[]string{"event"},
+	)
+
+	pathOverflow := prometheus.NewCounter(
+		prometheus.CounterOpts{
+			Name: "web_path_overflow_total",
+			Help: "Paths rejected due to cardinality limit",
+		},
+	)
+
+	return &MetricsAggregator{
+		registry:     registry,
+		cfg:          cfg,
+		pageviews:    pageviews,
+		customEvents: customEvents,
+		pathOverflow: pathOverflow,
+	}
+}
+
+// Records a pageview with the configured dimensions
+func (m *MetricsAggregator) RecordPageview(path, country, device, referrer string) {
+	// Build label values in the same order as label names
+	labels := prometheus.Labels{"path": path}
+
+	if m.cfg.Site.Collect.Country {
+		labels["country"] = country
+	}
+
+	if m.cfg.Site.Collect.Device {
+		labels["device"] = device
+	}
+
+	if m.cfg.Site.Collect.Referrer != "off" {
+		labels["referrer"] = referrer
+	}
+
+	m.pageviews.With(labels).Inc()
+}
+
+// Records a custom event
+func (m *MetricsAggregator) RecordCustomEvent(eventName string) {
+	m.customEvents.With(prometheus.Labels{"event": eventName}).Inc()
+}
+
+// Records a path that was rejected due to cardinality limits
+func (m *MetricsAggregator) RecordPathOverflow() {
+	m.pathOverflow.Inc()
+}
+
+// Registers all metrics with the provided Prometheus registry
+func (m *MetricsAggregator) MustRegister(reg prometheus.Registerer) {
+	reg.MustRegister(m.pageviews)
+	reg.MustRegister(m.customEvents)
+	reg.MustRegister(m.pathOverflow)
+}