Merge #4295

4295: Enable Glean docs for engine-gecko-nightly r=Dexterp37 a=Dexterp37

This additionally changes the GV updater bot to add doc changes to the PR it creates when updating GV.


**Note**: how can I test the script on CI?
Co-authored-by: Alessio Placitelli <alessio.placitelli@gmail.com>

automation/taskcluster/action/update_geckoview.sh

@@ -40,19 +40,26 @@ git commit -m \
 	--author="MickeyMoz <sebastian@mozilla.com>" \
 || { echo "No new GeckoView version ($CHANNEL) available"; exit 0; }
 
+# This will be 'engine-gecko-$CHANNEL' for non-release channels.
+# Intentionally left empty.
+COMPONENT_NAME=
+
 # Build and test engine component as well as sample browser
 if [ "$CHANNEL" = "nightly" ]
 then
+    COMPONENT_NAME="engine-gecko-nightly"
     ./gradlew browser-engine-gecko-nightly:assemble \
           browser-engine-gecko-nightly:test \
           sample-browser:assembleGeckoNightlyUniversal
 elif [ "$CHANNEL" = "beta" ]
 then
+    COMPONENT_NAME="engine-gecko-beta"
     ./gradlew browser-engine-gecko-beta:assemble \
           browser-engine-gecko-beta:test \
           sample-browser:assembleGeckoBetaUniversal
 elif [ "$CHANNEL" = "release" ]
 then
+    COMPONENT_NAME="engine-gecko"
     ./gradlew browser-engine-gecko:assemble \
           browser-engine-gecko:test \
           sample-browser:assembleGeckoReleaseArm
@@ -61,6 +68,12 @@ else
     exit 1
 fi
 
+# The build system automatically updates the docs. Add them to the
+# docs PR, if they exist.
+if git add components/browser/${COMPONENT_NAME}/docs/metrics.md ; then
+    git commit --no-edit --amend
+fi
+
 # Get token for using GitHub
 python automation/taskcluster/helper/get-secret.py \
     -s project/mobile/github \

components/browser/engine-gecko-nightly/build.gradle

@@ -34,6 +34,8 @@ android {
 // file from AAR dependencies of this project rather than look
 // for it into the project directory.
 ext.allowMetricsFromAAR = true
+// Generate markdown docs for the collected Gecko metrics.
+ext.gleanGenerateMarkdownDocs = true
 
 dependencies {
     implementation project(':concept-engine')

components/browser/engine-gecko-nightly/docs/metrics.md

+<!-- AUTOGENERATED BY glean_parser.  DO NOT EDIT. -->
+
+# Metrics
+This document enumerates the metrics collected by this project.
+This project may depend on other projects which also collect metrics.
+This means you might have to go searching through the dependency tree to get a full picture of everything collected by this project.
+Sorry about that.
+
+# Pings
+
+ - [metrics](#metrics)
+
+
+## metrics
+This is a built-in ping that is assembled out of the box by the Glean SDK.
+See the Glean SDK documentation for the [`metrics` ping](https://mozilla.github.io/glean/book/user/pings/metrics.html).
+The following metrics are added to the ping:
+
+| Name | Type | Description | Data reviews | Extras | Expiration |
+| --- | --- | --- | --- | --- | --- |
+| test.glean.geckoview.streaming |[timing_distribution](https://mozilla.github.io/glean/book/user/metrics/timing_distribution.html) |A test-only, disabled metric. This is required to guarantee that a `GleanGeckoHistogramMapping` is always generated, even though the GeckoView AAR exports no metric. Please note that the data-review field below contains no review, since this metric is disabled and not allowed to collect any data.  |[1](https://bugzilla.mozilla.org/show_bug.cgi?id=1566374)||never |
+
+
+<!-- AUTOGENERATED BY glean_parser.  DO NOT EDIT. -->
+