[APM] Test plan proposal (#44887)

* [APM] Test plan proposal

* Fix version label

Co-Authored-By: Gil Raphaelli <gil@elastic.co>

* Update TEST_PLAN.md

x-pack/legacy/plugins/TEST_PLAN.md

@@ -0,0 +1,27 @@
+# Test plan process (7.4.0)
+
+### What
+Everything that is being released in the 7.4 should be tested by someone other than the person who implemented it 
+
+### When
+The first week after feature freeze (FF) all known bugs and blockers will be fixed. The test plan will begin shortly after this (typically one week after FF date)
+
+### Prerequesites
+ - Every pull request that is merged to 7.x (or backported to 7.4 after FF) must be given the label: `v7.4.0`. [These pull requests](https://github.com/elastic/kibana/pulls?utf8=%E2%9C%93&q=is%3Apr+label%3Av7.4.0+label%3ATeam%3Aapm+-label%3Abackport) will provide the basis of our test plan. 
+ - Immediately before starting the test plan all relevant PRs for the release are selected and the label `in-test-plan` is applied. The "assigned" field will be reset to nobody (read further why)
+
+### Execution
+ - [Find the list](https://github.com/elastic/kibana/pulls?q=is%3Apr+label%3Av7.4.0+label%3Ain-test-plan+label%3ATeam%3Aapm+-label%3Atested-after-ff+no%3Aassignee) of unassigned, untested issues in the test plan
+ - Assign yourself to an issue as you start testing it to avoid duplicate work
+ - Testing a PR can have two outcomes:
+   - It worked as expected:
+     - Apply `tested-after-ff` label
+   - It did not work as expected: 
+     - Create a new issue: 
+        - Link back to the original PR
+        - Add the version labels to the issue and on Zube move it to "Scheduled for release"-column
+     - The original PR should be marked as tested with `tested-after-ff` (the new issue will ensure that the bug is prioritized and later tested)
+
+
+
+