Fix build

Author: jack-berg

exporters/sender/okhttp/src/main/java/io/opentelemetry/exporter/sender/okhttp/internal/OkHttpGrpcSenderProvider.java

@@ -22,7 +22,7 @@ public GrpcSender createSender(GrpcSenderConfig grpcSenderConfig) {
     return new OkHttpGrpcSender(
         grpcSenderConfig
             .getEndpoint()
-            .resolve(grpcSenderConfig.getServiceAndMethodName())
+            .resolve("/" + grpcSenderConfig.getServiceAndMethodName())
             .toString(),
         grpcSenderConfig.getCompressor(),
         grpcSenderConfig.getTimeoutNanos(),