Make the ZipkinSpanExporter public, so it can actually be used. (#1186) (#1228)

(cherry picked from commit 414ddb0)

Co-authored-by: John Watson <[email protected]>

Author: bogdandrutu

exporters/zipkin/src/main/java/io/opentelemetry/exporters/zipkin/ZipkinSpanExporter.java

@@ -47,7 +47,7 @@
  * This class was based on the OpenCensus zipkin exporter code at
  * https://github.com/census-instrumentation/opencensus-java/tree/c960b19889de5e4a7b25f90919d28b066590d4f0/exporters/trace/zipkin
  */
-final class ZipkinSpanExporter implements SpanExporter {
+public final class ZipkinSpanExporter implements SpanExporter {
 
   private static final Logger logger = Logger.getLogger(ZipkinSpanExporter.class.getName());