Sentry: guidelines on how to create performance dashboards and performance alerts (#129)

This PR includes guidelines on how to:
- [Setup a Sentry dashboard, to visualise the evolution of a given
performance metric over
time.](https://github.com/MetaMask/contributor-docs/blob/4878af2b48bc36d4d4aa347b82e2c73855487552/docs/performance-dashboard.md)
- [Setup a Sentry alert, to be notified in case a given performance
metric regresses over
time.](https://github.com/MetaMask/contributor-docs/blob/4878af2b48bc36d4d4aa347b82e2c73855487552/docs/performance-alert.md)

Author: gauthierpetetin

README.md

@@ -10,6 +10,8 @@ This is a living repository — nothing is set in stone! If you're member of Met
 - [Engineering Principles](./docs/engineering-principles.md)
 - [JavaScript Guidelines](./docs/javascript.md)
 - [Migrations Best Practices](./docs/migrations-guidelines.md)
+- [Performance Alert Guidelines](./docs/performance-alert.md)
+- [Performance Dashboard Guidelines](./docs/performance-dashboard.md)
 - [Performance Tracing Guidelines](./docs/performance-tracing.md)
 - [Pull Requests Guide](./docs/pull-requests.md)
 - [React Guidelines](./docs/react.md)

docs/images/performance-alert-actions.png

@@ -0,0 +1,100 @@
+# Performance Alert
+
+## Overview
+
+Performance alerts can be created in Sentry to get notified in case a given performance metric regresses over time.
+
+## Choose the trace you want to monitor
+
+The performance metric needed to monitor the performance of a given user flow can be tracked by adding a performance trace to the codebase (or by using an existing trace if one already exists). More details on performance tracing can be found [here](./performance-tracing.md).
+
+## Create a new alert
+
+Go to [Sentry alerts page](https://metamask.sentry.io/alerts/rules/).
+
+Click on "Create Alert".
+
+Select the `Performance > Transaction duration` as alert type.
+
+![performance-alert-types](./images/performance-alert-types.png)
+
+Click on "Set Conditions".
+
+### 1. Define your metric
+
+Leave this section unchanged, except the `p95` that you can change into `p75`.
+
+![performance-alert-metric](./images/performance-alert-metric.png)
+
+### 2. Filter events
+
+Indicate the name of the project:
+
+- Choose `metamask` project for Extension
+- Choose `metamask-mobile` project for Mobile
+
+Indicate the name of the environment: `production`.
+
+For Extension, use the following query format:
+
+```
+transaction:"name-of-the-trace"
+```
+
+For Mobile, if you want to filter for iOS metrics only, use the following query format:
+
+```
+transaction:"name-of-the-trace" release.package:io.metamask.MetaMask
+```
+
+For Mobile, if you want to filter for Android metrics only, use the following query format:
+
+```
+transaction:"name-of-the-trace" release.package:io.metamask
+```
+
+Example filter:
+
+```
+transaction:"UI Startup" release.package:io.metamask.MetaMask
+```
+
+### 3. Set thresholds
+
+Select the following options:
+
+- Type: `Anomaly`
+- Level of responsiveness `Medium`
+- Direction of anomaly movement: `Above bounds only`
+
+![performance-alert-thresholds](./images/performance-alert-thresholds.png)
+
+### 4. Set actions
+
+Click on "Add Action".
+
+Select the following options:
+
+- Action type: `Critical Status`
+- Channel type: `Slack - Consensys`
+- Slack channel name: `#your-slack-channel-name` (Example: #metamask-your-team-name-release-monitoring)
+- Slack channel ID: `your-slack-channel-id` (Example: C085BL3Z04J)
+
+![performance-alert-actions](./images/performance-alert-actions.png)
+
+PS: We recommend that you create a Slack channel called #metamask-your-team-name-release-monitoring, specifically for these Sentry alerts.
+PSS: Here's an [article](https://zeroheight.com/help/article/retrieving-a-slack-channel-id-from-slack/) explaining how to retrieve the ID of a Slack channel.
+
+### 5. Establish ownership
+
+Choose a self explanatory name for your alert, like: `Anomaly - name-of-the-trace (type-of-application)`.
+
+Example: `Anomaly - UI Startup (iOS)` or `Anomaly - UI Startup (Android)`
+
+Select `#metamask`.
+
+Click on "Save Rule".
+
+That's it. Your performance alert is now configured and will post messages to the Slack channel you chose.
+
+![performance-alert-slack-message](./images/performance-alert-slack-message.png)