Finagle Toggles - Export toggle value metric

Adding toggle value to observability data to enable the
Integration between [[ https://docbird.twitter.biz/precog/interfaces/instigator/FTInstigator.html |  Precog FT ]] and  [[ https://docs.google.com/document/d/1NwnQMOWeZp7Ys3wUePTdDrx3fQ3vTtMaHo22T9zBYz4/edit#heading=h.bz9cvnszgqss | Core Entity Service: Graceful Degradation
 ]]
 probing the toggle value using Observability metrics to avoid calling admin point for every shard

Differential Revision: https://phabricator.twitter.biz/D1138842

finagle-toggle/src/main/scala/com/twitter/finagle/toggle/ToggleMap.scala

@@ -11,7 +11,8 @@ import java.util.{function => juf}
 import java.{lang => jl}
 import scala.annotation.varargs
 import scala.collection.JavaConverters._
-import scala.collection.{immutable, mutable}
+import scala.collection.immutable
+import scala.collection.mutable
 import scala.util.hashing.MurmurHash3
 
 /**
@@ -170,6 +171,9 @@ object ToggleMap {
         }
         crc32.getValue.toFloat
       }
+      private[this] val toggleValue = statsReceiver.addGauge("fraction") {
+        iterator.toIndexedSeq.headOption.map(_.fraction.toFloat).getOrElse(0)
+      }
 
       def underlying: ToggleMap = toggleMap
 

finagle-toggle/src/test/scala/com/twitter/finagle/toggle/ToggleMapTest.scala

@@ -1,7 +1,11 @@
 package com.twitter.finagle.toggle
 
-import com.twitter.finagle.stats.{InMemoryStatsReceiver, NullStatsReceiver}
-import com.twitter.logging.{BareFormatter, Level, Logger, StringHandler}
+import com.twitter.finagle.stats.InMemoryStatsReceiver
+import com.twitter.finagle.stats.NullStatsReceiver
+import com.twitter.logging.BareFormatter
+import com.twitter.logging.Level
+import com.twitter.logging.Logger
+import com.twitter.logging.StringHandler
 import org.scalacheck.Arbitrary.arbitrary
 import org.scalacheck.Gen
 import org.scalatestplus.scalacheck.ScalaCheckDrivenPropertyChecks
@@ -44,6 +48,32 @@ class ToggleMapTest extends AnyFunSuite with ScalaCheckDrivenPropertyChecks with
     assert(state2 == gauge())
   }
 
+  test("ToggleMap.observed produces a toggleValue summary") {
+    val stats = new InMemoryStatsReceiver()
+    val inMem = ToggleMap.newMutable()
+    val map = ToggleMap.observed(inMem, stats)
+
+    val gauge: () => Float = stats.gauges(Seq("fraction"))
+
+    val initial = 0f
+    val state1 = 0.5f
+    val state2 = 0.1f
+
+    // without changes
+    assert(initial == gauge())
+
+    val toggleName = "com.id"
+
+    // validate fraction changes after an add
+    inMem.put(toggleName, 0.5)
+    assert(state1 == gauge())
+
+    // validate fraction changes after an update
+    inMem.put(toggleName, 0.1)
+    assert(state2 == gauge())
+
+  }
+
   test("ToggleMap.observed produces Toggle.Captured") {
     val inMem = ToggleMap.newMutable()
     val tm = ToggleMap.observed(inMem, NullStatsReceiver)