chore: import upstream snapshot with attribution

pkg/observability/metrics/metrics.go

@@ -0,0 +1,13 @@
+package metrics
+
+import "time"
+
+// Metrics is a backend-agnostic interface for emitting metrics.
+// Implementations can route to DataDog, log to slog, or discard (noop).
+type Metrics interface {
+	Increment(key string, tags map[string]string)
+	Counter(key string, tags map[string]string, value int64)
+	Distribution(key string, tags map[string]string, value float64)
+	DistributionMs(key string, tags map[string]string, value time.Duration)
+	WithTags(tags map[string]string) Metrics
+}