Codecov allow 0.2% coverage threshold (#3553)

- Add 0.2% code coverage threshold to account
 for flaky tests.
- Ref: https://docs.codecov.com/docs/commit-status#threshold
- "Allow the coverage to drop by X%, and posting a success status."

codecov.yml

@@ -1,9 +1,16 @@
+# Ref: https://docs.codecov.com/docs/codecov-yaml
 coverage:
   precision: 2
   round: nearest
   range: "40...70"
   ignore:
     - "generator/src/main/java/**/*"
     - "samples/**/*"
+  # Ref: https://docs.codecov.com/docs/commit-status
   status:
-    patch: off
+    patch:
+      off
+    project:
+      default:
+        target: auto
+        threshold: 0.2%