Bug 1566352 - Document GeckoView Streaming r=janerik

Differential Revision: https://phabricator.services.mozilla.com/D38739

--HG--
extra : moz-landing-system : lando

toolkit/components/telemetry/docs/internals/geckoview-streaming.rst

@@ -0,0 +1,14 @@
+GeckoView Streaming API
+=======================
+
+As an alternative to `GeckoView <../internals/geckoview>` mode,
+Telemetry can instead route Histogram accumulations out of Gecko to a Telemetry Delegate.
+
+To do this, ``toolkit.telemetry.geckoview.streaming`` must be set to true,
+and Gecko must have been built with ``MOZ_WIDGET_ANDROID`` defined.
+
+Details
+=======
+
+Accumulations from Histograms that have a ``products`` list that includes ``geckoview_streaming`` will be redirected to a small batching service in ``toolkit/components/telemetry/geckoview/streaming``.
+The batching service (essentially just a table of histogram names to lists of samples) will hold on to these lists of accumulations paired to the histogram names for a length of time (``toolkit.telemetry.geckoview.batchDurationMS`` (default 5000)) after which the next accumulation will trigger the whole batch (all lists) to be passed over to the ``StreamingTelemetryDelegate``.

toolkit/components/telemetry/docs/internals/preferences.rst

@@ -233,12 +233,22 @@ GeckoView
 
 ``toolkit.telemetry.isGeckoViewMode``
 
-   Whether or not Telemetry needs to run in :doc:`GeckoView <../internals/geckoview>` mode. If true, measurements persistence is enabled. Defaults to false on all products except GeckoView.
+   Whether or not Telemetry needs to run in :doc:`GeckoView <../internals/geckoview>` mode. If true, and ``toolkit.telemetry.geckoview.streaming`` is false,  measurements persistence is enabled. Defaults to false on all products except GeckoView.
 
 ``toolkit.telemetry.geckoPersistenceTimeout``
 
    The interval that governs how frequently measurements are saved to disk, in milliseconds. Defaults to 60000 (60 seconds).
 
+``toolkit.telemetry.geckoview.streaming``
+
+   Whether the GeckoView mode we're running in is the variety that uses the :doc:`GeckoView Streaming Telemetry API <../internals/geckoview-streaming>` or not.
+   Defaults to false.
+
+``toolkit.telemetry.geckoview.batchDurationMS``
+
+   The duration in milliseconds over which `GeckoView Streaming Telemetry <../internals/geckoview-streaming>` will batch accumulations before passing it on to its delegate.
+   Defaults to 5000.
+
 Testing
 -------