Update zipkin-reporter to the latest version 2.3.2. (#1000)

Author: Bogdan Drutu

build.gradle

@@ -140,8 +140,8 @@ subprojects {
         googleAuthVersion = '0.9.0'
         googleCloudVersion = '0.34.0-beta'
         signalfxVersion = '0.0.39'
-        zipkinReporterVersion = '2.0.0'
         prometheusVersion = '0.2.0'
+        zipkinReporterVersion = '2.3.2'
 
         libraries = [
                 auto_value: "com.google.auto.value:auto-value:${autoValueVersion}",

exporters/trace/zipkin/src/main/java/io/opencensus/exporter/trace/zipkin/ZipkinExporterHandler.java

@@ -114,6 +114,8 @@ static Span generateSpan(SpanData spanData, Endpoint localEndpoint) {
     @SuppressWarnings("nullness")
     long endTimestamp = toEpochMicros(spanData.getEndTimestamp());
 
+    // TODO(bdrutu): Fix the Checker Framework warning.
+    @SuppressWarnings("nullness")
     Span.Builder spanBuilder =
         Span.newBuilder()
             .traceId(encodeTraceId(context.getTraceId()))