finagle-thrift: Fill in ServiceClass stack param on clients and servers

Problem

Thrift's `ServiceClass` stack param is generally useful outside of Finagle's own codebase. Modules developed
externally may need access to this stack param, which is currently package-private.

Solution

Make `ServiceClass` stack param public.

JIRA Issues: CSL-10941

Differential Revision: https://phabricator.twitter.biz/D676781

CHANGELOG.rst

@@ -23,6 +23,10 @@ New Features
 * finagle-netty4: Added a new counter to keep track of the number of TLS connections that were
   started via snooping. ``PHAB_ID=D667652``
 
+* finagle-thrift: Thrift(Mux) clients and servers now fill in a `c.t.f.Thrift.param.ServiceClass`
+  stack param with the runtime class corresponding to a IDL-generated service stub.
+  ``PHAB_ID=D676781``
+
 Breaking API Changes
 ~~~~~~~~~~~~~~~~~~~~
 

finagle-thrift/src/main/scala/com/twitter/finagle/Thrift.scala

@@ -212,8 +212,8 @@ object Thrift
       val default = PerEndpointStats(false)
     }
 
-    private[finagle] final case class ServiceClass(clazz: Option[Class[_]])
-    private[finagle] implicit object ServiceClass extends Stack.Param[ServiceClass] {
+    final case class ServiceClass(clazz: Option[Class[_]])
+    implicit object ServiceClass extends Stack.Param[ServiceClass] {
       val default = ServiceClass(None)
     }
   }