Eagerly reject null label values

Scraping generally doesn't support null label values and can throw NPEs at various points. It's easiest to debug such problems at the point null is introduced.

Signed-off-by: Benjamin Peterson <[email protected]>

Author: benjaminp

prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/StatefulMetric.java

@@ -112,7 +112,20 @@ public D labelValues(String... labelValues) {
             "Expected " + labelNames.length + " label values, but got " + labelValues.length + ".");
       }
     }
-    return data.computeIfAbsent(Arrays.asList(labelValues), l -> newDataPoint());
+    return data.computeIfAbsent(
+        Arrays.asList(labelValues),
+        l -> {
+          for (int i = 0; i < l.size(); i++) {
+            if (l.get(i) == null) {
+              throw new IllegalArgumentException(
+                  "null label value for metric "
+                      + getMetadata().getName()
+                      + " and label "
+                      + labelNames[i]);
+            }
+          }
+          return newDataPoint();
+        });
   }
 
   /**