Add GitHub issue template for reporting flaky tests (apache#9398)

Co-authored-by: Sijie Guo <[email protected]>

.github/ISSUE_TEMPLATE/flaky_test.md

@@ -0,0 +1,33 @@
+---
+name: Flaky test
+about: Report a flaky test failure
+title: 'Flaky-test: [test class].[test method]'
+labels: component/test flaky-tests type/bug
+assignees: ''
+---
+<!--- 
+
+Instructions for reporting a flaky test using this issue template:
+
+1. Replace [test class] in title and body with the test class name
+2. Replace [test method] in title and body with the test method that failed. Multiple methods are flaky, remove the content that refers to the test method.
+3. Replace "url here" with a url to an example failure. In the Github Actions workflow run logs, you can right click on the line number to copy a link to the line. Example of such url is https://github.com/apache/pulsar/pull/8892/checks?check_run_id=1531075794#step:9:377 . The logs are available for a limited amount of time (usually for a few weeks).
+4. Replace "relevant parts of the exception stacktrace here" with the a few lines of the stack trace that shows at leat the exception message and the line of test code where the stacktrace occured.
+5. Replace "full exception stacktrace here" with the full exception stacktrace from logs. This section will be hidded by default.
+6. Remove all unused fields / content to unclutter the reported issue. Remove this comment too.
+
+-->
+[test class] is flaky. The [test method] test method fails sporadically.
+
+[example failure](url here)
+
+```
+[relevant parts of the exception stacktrace here]
+```
+
+<details>
+<summary>Full exception stacktrace</summary>
+<code><pre>
+full exception stacktrace here
+</pre></code>
+</details>