finagle-core: add "implementation" label to DefaultStatsReceiver

Problem:
We want to indicate which metrics are added by users as custom,
application-specific metrics.

Solution:
Add the scope "app" to DefaultStatsReceiver. Add the label
`implementation="app"`. All custom metrics will look like
```
app_serviceA_metricB {implementation="app"}
```

JIRA Issues: CSL-12028

Differential Revision: https://phabricator.twitter.biz/D901903

Author: tigerlily-he

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

@@ -23,7 +23,11 @@ object LoadedStatsReceiver extends StatsReceiverProxy {
  * [[com.twitter.finagle.util.LoadService]] mechanism.
  */
 object DefaultStatsReceiver extends StatsReceiverProxy {
-  def self: StatsReceiver = LoadedStatsReceiver
+  val self: StatsReceiver =
+    LoadedStatsReceiver
+      .dimensionalScope("app")
+      .label("implementation", "app")
+
   override def repr: DefaultStatsReceiver.type = this
 
   def get: StatsReceiver = this

util-stats/src/test/scala/com/twitter/finagle/stats/DefaultStatsReceiverTest.scala

@@ -0,0 +1,15 @@
+package com.twitter.finagle.stats
+
+import org.scalatest.funsuite.AnyFunSuite
+
+class DefaultStatsReceiverTest extends AnyFunSuite {
+  test("DefaultStatsReceiver has app in dimensional scope and label") {
+    LoadedStatsReceiver.self = new InMemoryStatsReceiver
+    val fooCounter = DefaultStatsReceiver.counter("foo")
+    val identity = fooCounter.metadata.toMetricBuilder.get.identity
+
+    assert(identity.dimensionalName == Seq("app", "foo"))
+    assert(identity.hierarchicalName == Seq("foo"))
+    assert(identity.labels == Map("implementation" -> "app"))
+  }
+}