general metrics: require tags to be present

backend/src/appenlight/validators.py

@@ -480,7 +480,7 b' class GeneralMetricSchema(colander.MappingSchema):'
                                       missing='unknown')
     timestamp = colander.SchemaNode(NonTZDate(), validator=limited_date,
                                     missing=deferred_utcnow)
-    tags = TagSchemaList()
+    tags = TagSchemaList(missing=colander.required)
 
 
 class GeneralMetricsListSchema(colander.SequenceSchema):