util-stats: use service scopeSeparator in MetricBuilder

Problem
-------
Metric Metadata uses / as the scope separator in all cases. As a result, when service owners configure a custom scope separator (ex, -) the metric names will differ in metrics.json and metric_metadata.json.

Solution
--------
Create an object in util-stats to store a scope separating character for MetricBuilder and metric_metadata.json to make use of.

Result
------
Metric names now align across metrics.json and metric_metadata.json for services with custom configured scope separators

JIRA Issues: CSL-10202

Differential Revision: https://phabricator.twitter.biz/D557994

CHANGELOG.rst

@@ -7,6 +7,13 @@ Note that ``PHAB_ID=#`` and ``RB_ID=#`` correspond to associated messages in com
 Unreleased
 ----------
 
+Bug Fixes
+~~~~~~~~~
+* util-stat: `MetricBuilder` now uses a configurable metadataScopeSeparator to align
+  more closely with the metrics.json api. Services with an overridden scopeSeparator will
+  now see that reflected in metric_metadata.json where previously it was erroneously using
+  / in all cases. ``PHAB_ID=D557994``
+
 * util-slf4j-api: Better Java interop. Deprecate `c.t.util.logging.Loggers` as Java users should be
   able to use the `c.t.util.logging.Logger` companion object with less verbosity required.
   ``PHAB_ID=D558605``

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

@@ -12,6 +12,21 @@ case object NoRoleSpecified extends SourceRole
 case object Client extends SourceRole
 case object Server extends SourceRole
 
+/**
+ * finagle-stats has configurable scope separators. As this package is wrapped by finagle-stats, we
+ * cannot retrieve it from finagle-stats. Consequently, we created this object so that the
+ * scope-separator can be passed in for stringification of the MetricBuilder objects.
+ */
+object metadataScopeSeparator {
+  @volatile private var separator: String = "/"
+
+  def apply(): String = separator
+
+  private[finagle] def setSeparator(separator: String): Unit = {
+    this.separator = separator
+  }
+}
+
 /**
  * A builder class used to configure settings and metadata for metrics prior to instantiating them.
  * Calling any of the three build methods (counter, gauge, or histogram) will cause the metric to be
@@ -182,7 +197,7 @@ class MetricBuilder(
   }
 
   override def toString(): String = {
-    val nameString = name.mkString("/")
+    val nameString = name.mkString(metadataScopeSeparator())
     s"MetricBuilder($keyIndicator, $description, $units, $role, $verbosity, $sourceClass, $nameString, $relativeName, $processPath, $percentiles, $statsReceiver)"
   }
 }

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

@@ -0,0 +1,16 @@
+package com.twitter.finagle.stats
+
+import org.scalatest.funsuite.AnyFunSuite
+
+class MetricBuilderTest extends AnyFunSuite {
+
+  test("metadataScopeSeparator affects stringification") {
+    val mb = new MetricBuilder(name = Seq("foo", "bar"), statsReceiver = new InMemoryStatsReceiver)
+    assert(mb.toString.contains("foo/bar"))
+    metadataScopeSeparator.setSeparator("-")
+    assert(mb.toString.contains("foo-bar"))
+    metadataScopeSeparator.setSeparator("_aTerriblyLongStringForSomeReason_")
+    assert(mb.toString.contains("foo_aTerriblyLongStringForSomeReason_bar"))
+  }
+
+}