feat: add bounded operational metrics

internal/observability/health_test.go

@@ -5,6 +5,7 @@ import (
 	"net/http/httptest"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestHealthHandlerReadinessAndMetrics_HP_OPS_01(t *testing.T) {
@@ -35,8 +36,36 @@ func TestHealthMetricNamesAreBounded_BH_OPS_01(t *testing.T) {
 	t.Parallel()
 	health := New()
 	health.Inc("bad name")
+	health.Inc("1bad_name")
 	_, _, counters := health.Snapshot()
 	if len(counters) != 0 {
 		t.Fatalf("invalid metric name recorded: %#v", counters)
 	}
 }
+
+func TestHealthRendersBoundedGaugesAndDurationHistograms_HP_OPS_02(t *testing.T) {
+	t.Parallel()
+	health := New()
+	health.SetGauge("client_spool_bytes", 4096)
+	health.SetGauge("bad gauge", 1)
+	health.ObserveDuration("dispatch_latency", 12*time.Millisecond)
+	health.ObserveDuration("bad latency", time.Second)
+
+	metrics := httptest.NewRecorder()
+	health.Handler(Paths{}).ServeHTTP(metrics, httptest.NewRequest(http.MethodGet, "/metrics", nil))
+	body := metrics.Body.String()
+	for _, want := range []string{
+		"rvbox_client_spool_bytes 4096",
+		"rvbox_dispatch_latency_seconds_bucket{le=\"0.05\"} 1",
+		"rvbox_dispatch_latency_seconds_bucket{le=\"+Inf\"} 1",
+		"rvbox_dispatch_latency_seconds_sum 0.012",
+		"rvbox_dispatch_latency_seconds_count 1",
+	} {
+		if !strings.Contains(body, want) {
+			t.Fatalf("metrics missing %q:\n%s", want, body)
+		}
+	}
+	if strings.Contains(body, "bad gauge") || strings.Contains(body, "bad latency") {
+		t.Fatalf("unbounded metric name rendered: %s", body)
+	}
+}