For mozilla-mobile#8803: add StartupTimeline ping type and gecko metric.

app/metrics.yaml

@@ -1856,3 +1856,21 @@ installation:
     notification_emails:
       - fenix-core@mozilla.com
     expires: "2020-09-01"
+
+startup.timeline:
+  gecko_init_main_thread_initial:
+    send_in_pings:
+      - startup-timeline
+    type: timespan
+    time_unit: nanosecond
+    description: >
+      The initial duration Gecko initialization blocks the main thread before
+      continuing on a background thread.
+    bugs:
+      - https://github.com/mozilla-mobile/fenix/issues/8803
+    data_reviews:
+      - TODO
+    notification_emails:
+      - perf-android-fe@mozilla.com
+      - esmyth@mozilla.com
+    expires: "2020-09-01"

app/pings.yaml

@@ -29,3 +29,19 @@ installation:
     - https://github.com/mozilla-mobile/fenix/pull/8074#issuecomment-586512202
   notification_emails:
     - fenix-core@mozilla.com
+
+startup-timeline:
+  description: >
+    This ping is intended to provide an understanding of startup performance.
+
+    The ping is intended to be captured by performance testing automation to report results
+    there, in addition to user telemetry. We place these metrics into their own ping in order
+    to isolate them and make this process easier.
+  include_client_id: false
+  bugs:
+    - https://github.com/mozilla-mobile/fenix/issues/8803
+  data_reviews:
+    - TODO
+  notification_emails:
+    - perf-android-fe@mozilla.com
+    - esmyth@mozilla.com

docs/metrics.md

@@ -12,6 +12,7 @@ This means you might have to go searching through the dependency tree to get a f
  - [events](#events)
  - [installation](#installation)
  - [metrics](#metrics)
+ - [startup-timeline](#startup-timeline)
 
 
 ## activation
@@ -214,6 +215,18 @@ The following metrics are added to the ping:
 | search.default_engine.name |[string](https://mozilla.github.io/glean/book/user/metrics/string.html) |If the search engine is pre-loaded with Fenix this value will be the search engine name. If it's a custom search engine (defined: https://github.com/mozilla-mobile/fenix/issues/1607) the value will be "custom"  |[1](https://github.com/mozilla-mobile/fenix/pull/1606), [2](https://github.com/mozilla-mobile/fenix/pull/5216)||2020-09-01 |
 | search.default_engine.submission_url |[string](https://mozilla.github.io/glean/book/user/metrics/string.html) |If the search engine is pre-loaded with Fenix this value will be he base URL we use to build the search query for the search engine. For example: https://mysearchengine.com/?query=%s. If it's a custom search engine (defined: https://github.com/mozilla-mobile/fenix/issues/1607) the value will be "custom"  |[1](https://github.com/mozilla-mobile/fenix/pull/1606), [2](https://github.com/mozilla-mobile/fenix/pull/5216)||2020-09-01 |
 
+## startup-timeline
+
+This ping is intended to provide an understanding of startup performance.
+The ping is intended to be captured by performance testing automation to report results there, in addition to user telemetry. We place these metrics into their own ping in order to isolate them and make this process easier.
+
+
+The following metrics are added to the ping:
+
+| Name | Type | Description | Data reviews | Extras | Expiration |
+| --- | --- | --- | --- | --- | --- |
+| startup.timeline.gecko_init_main_thread_initial |[timespan](https://mozilla.github.io/glean/book/user/metrics/timespan.html) |The initial duration Gecko initialization blocks the main thread before continuing on a background thread.  |[1](TODO)||2020-09-01 |
+
 
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