{finagle-core,util-stats}: Move LoadedStatsReceiver to util-stats from finagle-core

Problem

Putting LoadedStatsReceiver in finagle-core means that some things that finagle-core depends on
can't use LoadedStatsReceiver.

Solution

Let's push it down to util-stats, the only additional dependency we need is util-stats => util-app.

JIRA Issues: CSL-11393

Differential Revision: https://phabricator.twitter.biz/D763497

Author: mosesn

CHANGELOG.rst

@@ -14,6 +14,8 @@ New Features
   java.time.OffsetDateTime. `toInstant`, `toZonedDateTime`, and `toOffsetDateTime` also preserve
   nanosecond resolution. ``PHAB_ID=D757636``
 
+* util-stats: Moved `c.t.finagle.stats.LoadedStatsReceiver` and `c.t.finagle.stats.DefaultStatsReceiver`
+  from the finagle-core module to util-stats.  ``PHAB_ID=D763497``
 
 21.9.0
 ------

build.sbt

@@ -661,7 +661,7 @@ lazy val utilStats = Project(
           Seq("org.scala-lang.modules" %% "scala-parallel-collections" % "1.0.3" % "test")
       }
     }
-  ).dependsOn(utilCore, utilLint)
+  ).dependsOn(utilApp, utilCore, utilLint)
 
 lazy val utilTest = Project(
   id = "util-test",

util-stats/src/main/scala/com/twitter/finagle/stats/BUILD

@@ -16,6 +16,7 @@ scala_library(
         "3rdparty/jvm/com/fasterxml/jackson/module:jackson-module-scala",
         "3rdparty/jvm/com/github/ben-manes/caffeine",
         "3rdparty/jvm/com/google/code/findbugs:jsr305",
+        "util/util-app/src/main/scala",
         "util/util-core:util-core-util",
         "util/util-lint/src/main/scala/com/twitter/util/lint",
     ],

util-stats/src/main/scala/com/twitter/finagle/stats/LoadedStatsReceiver.scala

@@ -0,0 +1,30 @@
+package com.twitter.finagle.stats
+
+import com.twitter.app.LoadService
+
+/**
+ * A [[com.twitter.finagle.stats.StatsReceiver]] that loads
+ * all service-loadable receivers and broadcasts stats to them.
+ */
+object LoadedStatsReceiver extends StatsReceiverProxy {
+
+  /**
+   * Mutating this value at runtime after it has been initialized should be done
+   * with great care. If metrics have been created using the prior
+   * [[StatsReceiver]], updates to those metrics may not be reflected in the
+   * [[StatsReceiver]] that replaces it. In addition, histograms created with
+   * the prior [[StatsReceiver]] will not be available.
+   */
+  @volatile var self: StatsReceiver = BroadcastStatsReceiver(LoadService[StatsReceiver]())
+}
+
+/**
+ * A "default" StatsReceiver loaded by the
+ * [[com.twitter.finagle.util.LoadService]] mechanism.
+ */
+object DefaultStatsReceiver extends StatsReceiverProxy {
+  def self: StatsReceiver = LoadedStatsReceiver
+  override def repr: DefaultStatsReceiver.type = this
+
+  def get: StatsReceiver = this
+}