Proposal for performance priority labels

Current description

To help clarify priority of issues, from the perspective of their impact on availability and performance of GitLab.com.

Closes https://gitlab.com/gitlab-com/infrastructure/issues/1943

Original description

I do not think this is a priority right now, but it stems from https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/5373/diffs which I broke into smaller merge requests.

source/handbook/engineering/performance/index.html.md

@@ -160,6 +160,51 @@ revision, new revision, and ref (e.g. tag or branch) name.
 1. Sidekiq updates PostgreSQL
 1. Unicorn can now query PostgreSQL.
+
+## Availability and Performance Priority Labels
+{: #performance-labels}
+
+To clarify the priority of issues that relate to GitLab.com's availability and
+performance consider adding an _Availability and Performance Priority Label_,
+`~AP1` through `~AP3`. This is similar to what is in use in the Support and
+Security teams, they use `~SE` and `~SL` labels respectively to indicate
+priority.
+
+Use the following as a guideline to determine which Availability and Performance
+Priority label to use for bugs and feature proposals. Consider the _likelihood_
+and _urgency_  of the "scenario" that could result from this issue (not) being
+resolved.
+
+- **Urgency:**  _Examples_
+ - U1
+    - Outage likely within a month.
+    - Affects many team members and/or many GitLab.com users
+ - U2
+    - Outage likely within three months.
+    - Affects some team members and/or a few GitLab.com users
+ - U3
+    - Outage can happen, but not likely in next three months.
+    - Affects some team members but no GitLab.com users
+
+- **Impact:** _Examples_
+ - I1
+    - Outage of >= 25 minutes.
+    - Performance improvement (or avoiding degradation) of >= 100 ms expected.
+ - I2
+    - Outage of 5 - 25 minutes.
+    - Performance improvement (or avoiding degradation) of 10-100 ms expected.
+ - I3
+    - Outage of 0 - 5 minutes.
+    - Performance improvement (or avoiding degradation) of <= 10 ms expected.
+
+
+| **Urgency \ Impact**          | **I1 - High** | **I2 - Medium**  | **I3 - Low**   |
+|----------------------------|---------------|------------------|----------------|
+| **U1 - High**              | `AP1`         | `AP1`            | `AP2`          |
+| **U2 - Medium**            | `AP1`         | `AP2`            | `AP3`          |
+| **U3 - Low**               | `AP2`         | `AP3`            | `AP3`          |
+
+
 ## Database Performance
 Some general notes about parameters that affect database performance, at a very