chore: add description for on call priority levels for bug fixes (#6184)

Author: RogerHYang

internal_docs/on_call.md

@@ -20,6 +20,13 @@ Being a maintainer of Phoenix and Openinference means you are responsible for th
     - [New issues without comments](https://github.com/search?q=repo%3AArize-ai%2Fopeninference+-author%3Acephalization+-author%3Aaxiomofjoy+-author%3Arogerhyang+-author%3Amikeldking+-author%3Aanticorrelator+-author%3Ajgilhuly+state%3Aopen+comments%3A0&type=issues&ref=advsearch&s=created&o=desc)
 - Accept or Reject Pull Requests, Review them if acceptable - [Phoenix](https://github.com/Arize-ai/phoenix/pulls) - [Openinference](https://github.com/Arize-ai/openinference/pulls)
 
+### Priority Levels for Bug Fix
+
+- P0: Should fix ASAP. Drop other tasks.
+- P1: Should fix within one week.
+- P2: Should fix within the next sprint. There's workaround but the bug is high visibility.
+- P3: Backlog
+
 ## Tips
 
 - "Watch" phoenix and openinference for all activity on github. You will get notifications for everything, including discussions.