scrooge-adaptive: Add logging to adaptive scrooge

Problem

Basically, users want to understand which fields were actually deserialized by
adaptive scrooge.

Solution

Use debug log with information about fields which adaptive srooge will
deserialize.

RB_ID=922648

build.sbt

@@ -284,7 +284,8 @@ lazy val scroogeAdaptive = Project(
     "asm" % "asm" % "3.3.1",
     "asm" % "asm-commons" % "3.3.1",
     "asm" % "asm-util" % "3.3.1",
-    "com.twitter" % "libthrift" % libthriftVersion % "provided"
+    "com.twitter" % "libthrift" % libthriftVersion % "provided",
+    util("logging")
   )
 ).dependsOn(scroogeCore, scroogeGenerator % "test", scroogeSerializer)
 

scrooge-adaptive/src/main/scala/BUILD

@@ -12,6 +12,7 @@ scala_library(
     'scrooge/scrooge-core',
     'scrooge/scrooge-serializer',
     'util/util-codec/src/main/scala:scala',
+    'util/util-logging/src/main/scala:scala',
   ],
   exports=[
     '3rdparty/jvm/asm',

scrooge-adaptive/src/main/scala/com/twitter/scrooge/adapt/AdaptTrackingDecoder.scala

@@ -1,10 +1,13 @@
 package com.twitter.scrooge.adapt
 
+import com.twitter.logging.Logger
 import com.twitter.scrooge.{ThriftStruct, ThriftStructCodec}
 import java.lang.reflect.InvocationTargetException
 import java.util.concurrent.atomic.AtomicInteger
 
 private[adapt] object AdaptTrackingDecoder {
+  val logger = Logger(this.getClass)
+
   val AdaptSuffix = "$Adapt"
   val AdaptDecoderSuffix = "$AdaptDecoder"
   val DecodeMethodName = "decode"
@@ -65,8 +68,22 @@ private[adapt] class AdaptTrackingDecoder[T <: ThriftStruct](
     val useMapById = useMapByField.map { case (f, v) => (f.id, v) }
 
     if (allFieldsUsed(useMapById)) {
+      logger.ifDebug(s"Adaptive scrooge is using all fields for ${codec.metaData.structName} struct.")
       fallbackDecoder
     } else {
+      val namesToUse = useMapByName.collect {
+        case (name, true) => name
+      }
+
+      if (namesToUse.isEmpty) {
+        logger.ifDebug(
+          s"Adaptive scrooge isn't using fields for ${codec.metaData.structName} struct with settings: $settings"
+        )
+      } else {
+        logger.ifDebug(
+          s"Adaptive scrooge is using fields: ${namesToUse.mkString(",")} for ${codec.metaData.structName} struct with settings: $settings"
+        )
+      }
       buildAdaptiveDecoder(useMapByName, useMapById)
     }
   }