atlas: fix npe for grouped lwc gauges (#1064)

When performing a group by, if there is a datapoint that does
not have all of the keys used in the grouping there would be
an NPE. They will now get ignored similar to the behavior for
counters or when not delaying the gauge aggregation.

spectator-reg-atlas/src/main/java/com/netflix/spectator/atlas/impl/Evaluator.java

@@ -167,10 +167,15 @@ public EvalPayload eval(long timestamp) {
             Map<String, String> tags = idMapper.apply(entry.getKey());
             tags.putAll(commonTags);
             if (delayGaugeAggr && consolidator.isGauge()) {
-              Map<String, String> resultTags = new HashMap<>(expr.resultTags(tags));
-              resultTags.put("atlas.aggr", idHash(entry.getKey()));
-              double acc = expr.isCount() ? 1.0 : v;
-              metrics.add(new EvalPayload.Metric(subId, resultTags, acc));
+              // When performing a group by, datapoints missing tag used for the grouping
+              // should be ignored
+              Map<String, String> rs = expr.resultTags(tags);
+              if (rs != null) {
+                Map<String, String> resultTags = new HashMap<>(rs);
+                resultTags.put("atlas.aggr", idHash(entry.getKey()));
+                double acc = expr.isCount() ? 1.0 : v;
+                metrics.add(new EvalPayload.Metric(subId, resultTags, acc));
+              }
             } else {
               TagsValuePair p = new TagsValuePair(tags, v);
               aggregator.update(p);

spectator-reg-atlas/src/test/java/com/netflix/spectator/atlas/impl/EvaluatorTest.java

@@ -271,6 +271,18 @@ public void delayAggrGaugeGroupByCount() {
     }
   }
 
+  @Test
+  public void delayAggrGaugeGroupByMissingKey() {
+    List<Subscription> subs = new ArrayList<>();
+    subs.add(newSubscription("sum", ":true,:sum,(,foo,),:by"));
+
+    Evaluator evaluator = newEvaluator(true);
+    evaluator.sync(subs);
+    EvalPayload payload = evaluator.eval(0L, data("foo", 1.0, 2.0, 3.0));
+
+    Assertions.assertEquals(0, payload.getMetrics().size());
+  }
+
   @Test
   public void delayAggrGaugeMax() {
     List<Subscription> subs = new ArrayList<>();