feat: Added essential type for partial granularity traces (newrelic#3547)

Author: svetlanabrennan

lib/spans/span-event.js

@@ -190,6 +190,17 @@ class SpanEvent {
       if (entityRelationshipAttrs.length > 0) {
         return { attributes, customAttributes, dropSpan: false }
       }
+    } else if (partialGranularityMode === 'essential') {
+      const attributesToKeep = {}
+      for (const item in attributes) {
+        if (entityRelationshipAttrs.includes(item) || item.startsWith('error.')) {
+          attributesToKeep[item] = attributes[item]
+        }
+      }
+
+      if (Object.keys(attributesToKeep).length > 0) {
+        return { attributes: attributesToKeep, customAttributes: {}, dropSpan: false }
+      }
     }
 
     return { dropSpan: true }