Reinstate BaseSpan interface for backwards compatibility

opentracing-api/src/main/java/io/opentracing/BaseSpan.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright 2016-2017 The OpenTracing Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License
+ * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package io.opentracing;
+
+/**
+ * BaseSpan is carried over from 0.30.0 API to preserve backwards compatibility with instrumentation
+ * that was compiled against v0.30.0. Even if that instrumentation does not refer to BaseSpan directly,
+ * it is still required at runtime (https://github.com/opentracing/opentracing-java/issues/237).
+ * 
+ * Keeping this interface around allows for easier transition to v0.31.0.
+ * 
+ * @see Span
+ * @deprecated since v0.31.0
+ */
+@Deprecated
+public interface BaseSpan {}