Remove confusing SamplingTracer from finagle-zipkin

CHANGES

@@ -12,6 +12,9 @@ Deprecations
   * finagle-http: Deprecate channelBufferUsageTracker in favor of maxRequestSize.
     ``RB_ID=831233``
 
+  * finagle-zipkin: Deprecate `ZipkinTracer` in favor of `ScribeZipkinTracer`.
+    ``RB_ID=??``
+
 Breaking API Changes
 ~~~~~~~~~~~~~~~~~~~~
 
@@ -54,8 +57,8 @@ Breaking API Changes
   * finagle-memcached: `c.t.f.memcached.Client` now uses `c.t.bijection.Bijection`
     instead of `c.t.u.Bijection`. ``RB_ID=834383``
 
-  * finagle-zipkin: Moved case classes `Span`, `ZipkinAnnotation`,
-    `BinaryAnnotation`, `Endpoint` and `Sampler` to finagle-zipkin-core.
+  * finagle-zipkin: Moved case classes and companion objects `Span`, `ZipkinAnnotation`,
+    `BinaryAnnotation`, `Endpoint`, `Sampler` and `SamplingTracer` to finagle-zipkin-core.
     ``RB_ID=??``
 
   * Builds are now only for Java 8 and Scala 2.11. See the

finagle-zipkin/src/main/resources/META-INF/services/com.twitter.finagle.tracing.Tracer

@@ -1 +1 @@
-com.twitter.finagle.zipkin.thrift.SamplingTracer
+com.twitter.finagle.zipkin.thrift.ScribeZipkinTracer

finagle-zipkin/src/main/scala/com/twitter/finagle/zipkin/thrift/ZipkinTracer.scala → finagle-zipkin/src/main/scala/com/twitter/finagle/zipkin/thrift/ScribeZipkinTracer.scala

@@ -56,16 +56,22 @@ object ZipkinTracer {
 }
 
 /**
- * SamplingTracer with a default constructor for the service loader to create a Scribe backed tracer
+ * Receives the Finagle generated traces and sends a sample of them off to Zipkin via Scribe
  */
-class SamplingTracer extends core.SamplingTracer(
-  ScribeRawZipkinTracer(Host().getHostName, Host().getPort,
+class ScribeZipkinTracer(tracer: ScribeRawZipkinTracer, sampleRate: Float)
+  extends core.SamplingTracer(tracer, sampleRate) {
+  /**
+   * Default constructor for the service loader
+   */
+  def this() = this(ScribeRawZipkinTracer(Host().getHostName, Host().getPort,
     DefaultStatsReceiver.scope("zipkin")), sampleRateFlag())
+}
 
 /**
- * Receives the Finagle generated traces and sends them off to Zipkin via Scribe
+ * Receives the Finagle generated traces and sends a sample of them off to Zipkin via Scribe
  * @param tracer underlying tracer
  * @param sampleRate ratio of requests to trace
  */
+@deprecated("Use ScribeZipkinTracer instead", "6.36.0")
 class ZipkinTracer(tracer: ScribeRawZipkinTracer, sampleRate: Float)
   extends core.SamplingTracer(tracer, sampleRate)