fix(metrics): ignore negative Counter.Add to keep counters monotonic

Counter.Add applied any delta, so a negative value would decrement a counter that must be monotonic, corrupting rate()-style downstream math. Ignore delta<0. Adds TestCounterRejectsNegativeAdd.

internal/metrics/metrics.go

@@ -33,8 +33,13 @@ func (c *Counter) Inc() {
 	c.value.Add(1)
 }
 
-// Add adds the given value to the counter.
+// Add adds the given value to the counter. Counters are monotonic, so a
+// negative delta (which would corrupt rate()-style calculations downstream) is
+// ignored rather than applied.
 func (c *Counter) Add(delta int64) {
+	if delta < 0 {
+		return
+	}
 	c.value.Add(delta)
 }
 

internal/metrics/metrics_test.go

@@ -71,6 +71,16 @@ func TestCounter(t *testing.T) {
 	}
 }
 
+// Counters are monotonic: a negative Add must be ignored, not applied.
+func TestCounterRejectsNegativeAdd(t *testing.T) {
+	c := NewCounter("mono_counter", "test", nil)
+	c.Add(5)
+	c.Add(-3)
+	if got := c.Value(); got != 5 {
+		t.Errorf("negative Add should be ignored: Value() = %d, want 5", got)
+	}
+}
+
 func TestCounterConcurrency(t *testing.T) {
 	c := NewCounter("concurrent_counter", "test", nil)
 	var wg sync.WaitGroup