Require array or object as the root type in object metrics

While this could be considered breaking, it's not in practice.
E.g. the FOG code implementing object metrics already requires arrays/objects at the root.

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - New lint: error when there are metrics whose names are too similar ([bug 1934099](https://bugzilla.mozilla.org/show_bug.cgi?id=1934099))
+- Require `array` or `object` as the root type in object metrics ([#780](https://github.com/mozilla/glean_parser/pull/780))
 
 ## 16.1.0
 

glean_parser/metrics.py

@@ -509,6 +509,20 @@ def validate_structure(structure):
         if None:
             raise ValueError("`structure` needed for object metric.")
 
+        # Different from `ALLOWED_TYPES`:
+        # We _require_ the root type to be an object or array.
+        allowed_types = ["object", "array"]
+        if "type" not in structure:
+            raise ValueError(
+                f"missing `type` in object structure. Allowed: {allowed_types}"
+            )
+        if structure["type"] not in allowed_types:
+            raise ValueError(
+                "invalid `type` in object structure. found: {}, allowed: {}".format(
+                    structure["type"], allowed_types
+                )
+            )
+
         structure = Object._validate_substructure(structure)
         return structure
 

tests/test_parser.py

@@ -1273,3 +1273,11 @@ def test_object_invalid():
     errors = list(all_metrics)
     assert len(errors) == 1
     assert "`items` not allowed in object structure" in errors[0]
+
+    structure = {"type": "string"}
+    contents = [{"category": {"metric": {"type": "object", "structure": structure}}}]
+    contents = [util.add_required(x) for x in contents]
+    all_metrics = parser.parse_objects(contents)
+    errors = list(all_metrics)
+    assert len(errors) == 1
+    assert "invalid `type` in object structure." in errors[0]