finagle-core: Disable request logging by default

Problem / Solution

The cost of request logging, even when not being used,
outweighs the cost of having it on by default.

Result

Service owners can manually enable it using the `RequestLogger`
Stack param.

JIRA Issues: CSL-7729

Differential Revision: https://phabricator.twitter.biz/D308476

CHANGELOG.rst

@@ -14,6 +14,9 @@ Runtime Behavior Changes
   "loadband" scoped metrics: "widen", "narrow", "offered_load_ema". These were not
   necessary as d-aperture does not change the aperture size at runtime. ``PHAB_ID=D303833``
 
+* finagle-core: Request logging now defaults to disabled. Enable it by configuring the
+  `RequestLogger` Stack parameter on your `Client` or `Server`. ``PHAB_ID=D308476``
+
 Breaking API Changes
 ~~~~~~~~~~~~~~~~~~~~
 

doc/src/sphinx/FAQ.rst

@@ -224,7 +224,24 @@ Finagle's :ref:`clients  <client_modules>` and :ref:`servers <server_modules>`
 have many modules that are tasked with a wide assortment of jobs. When there
 is unexpected latency, it can be useful to have visibility into where time
 is spent. Finagle's `RequestLogger` can help with this. It can be enabled by
-setting the ``com.twitter.finagle.request.Logger`` level to ``TRACE``.
+setting the ``com.twitter.finagle.request.Logger`` level to ``TRACE`` and
+enabling the stack param:
+
+.. code-block:: scala
+
+  // scala
+  import com.twitter.finagle.filter.RequestLogger
+
+  Protocol.client.configured(RequestLogger.Enabled)
+  Protocol.server.configured(RequestLogger.Enabled)
+
+.. code-block:: java
+
+  // java
+  import com.twitter.finagle.filter.RequestLogger;
+
+  Protocol.client.configured(RequestLogger.Enabled().mk());
+  Protocol.server.configured(RequestLogger.Enabled().mk());
 
 The logs include synchronous and asynchronous time for each stack module's
 `Filter`. Synchronous here means the time spent from the beginning of the

finagle-core/src/main/scala/com/twitter/finagle/filter/RequestLogger.scala

@@ -23,16 +23,16 @@ object RequestLogger {
     case object Disabled extends Param
     case object Enabled extends Param
 
-    implicit val param: Stack.Param[Param] = Stack.Param(Param.Enabled)
+    implicit val param: Stack.Param[Param] = Stack.Param(Param.Disabled)
   }
 
   /**
-   * Enables the [[RequestLogger]] (enabled by default).
+   * Enables the [[RequestLogger]].
    */
   val Enabled: Param = Param.Enabled
 
   /**
-   * Disables the [[RequestLogger]].
+   * Disables the [[RequestLogger]] (disabled by default).
    */
   val Disabled: Param = Param.Disabled