atlas: aggr function with empty tags

Uses a placeholder name of `unknown` if the query for an
aggregate function has no exact tags. This matches the
behavior of the backend.

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

@@ -176,6 +176,16 @@ final class All implements DataExpr {
 
   /** Base type for simple aggregate functions. */
   interface AggregateFunction extends DataExpr {
+
+    /** Return the exact matches from the query clause. */
+    Map<String, String> queryTags();
+
+    @Override default Map<String, String> resultTags(Map<String, String> tags) {
+      Map<String, String> ts = queryTags();
+      return ts.isEmpty()
+          ? Collections.singletonMap("name", "unknown")
+          : ts;
+    }
   }
 
   /**
@@ -202,7 +212,7 @@ final class Sum implements AggregateFunction {
       return true;
     }
 
-    @Override public Map<String, String> resultTags(Map<String, String> tags) {
+    @Override public Map<String, String> queryTags() {
       return queryTags;
     }
 
@@ -269,7 +279,7 @@ final class Min implements AggregateFunction {
       return false;
     }
 
-    @Override public Map<String, String> resultTags(Map<String, String> tags) {
+    @Override public Map<String, String> queryTags() {
       return queryTags;
     }
 
@@ -336,7 +346,7 @@ final class Max implements AggregateFunction {
       return false;
     }
 
-    @Override public Map<String, String> resultTags(Map<String, String> tags) {
+    @Override public Map<String, String> queryTags() {
       return queryTags;
     }
 
@@ -407,7 +417,7 @@ final class Count implements AggregateFunction {
       return true;
     }
 
-    @Override public Map<String, String> resultTags(Map<String, String> tags) {
+    @Override public Map<String, String> queryTags() {
       return queryTags;
     }
 
@@ -496,7 +506,7 @@ private Map<String, String> keyTags(Map<String, String> tags) {
       if (resultTags == null) {
         return null;
       } else {
-        resultTags.putAll(af.resultTags(tags));
+        resultTags.putAll(af.queryTags());
         return resultTags;
       }
     }

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

@@ -313,6 +313,20 @@ public void mismatchedClosingParen() {
         () -> Parser.parseDataExpr("key,(,key,),),:in,:sum"));
   }
 
+  @Test
+  public void orClauseTags() {
+    DataExpr expr = parse("name,foo,:eq,name,bar,:eq,:or,:sum");
+    Map<String, String> tags = expr.resultTags(Collections.singletonMap("name", "foo"));
+    Assertions.assertEquals(Collections.singletonMap("name", "unknown"), tags);
+  }
+
+  @Test
+  public void orClauseTagsGroupBy() {
+    DataExpr expr = parse("name,foo,:eq,name,bar,:eq,:or,:sum,(,name,),:by");
+    Map<String, String> tags = expr.resultTags(Collections.singletonMap("name", "foo"));
+    Assertions.assertEquals(Collections.singletonMap("name", "foo"), tags);
+  }
+
   @Test
   public void allEqualsContract() {
     EqualsVerifier