Adds RSocket instrumentation

coauthor @olegdokuka

fixes gh-1677

spring-cloud-sleuth-api/src/main/java/org/springframework/cloud/sleuth/SpanAndScope.java

@@ -41,4 +41,9 @@ public class SpanAndScope {
 		return this.scope;
 	}
 
+	@Override
+	public String toString() {
+		return "SpanAndScope{" + "span=" + this.span + '}';
+	}
+
 }

spring-cloud-sleuth-api/src/main/java/org/springframework/cloud/sleuth/TraceContext.java

@@ -51,4 +51,47 @@ public interface TraceContext {
 	 */
 	Boolean sampled();
 
+	/**
+	 * Builder for {@link TraceContext}.
+	 *
+	 * @since 3.1.0
+	 */
+	interface Builder {
+
+		/**
+		 * Sets trace id on the trace context.
+		 * @param traceId trace id
+		 * @return this
+		 */
+		TraceContext.Builder traceId(String traceId);
+
+		/**
+		 * Sets parent id on the trace context.
+		 * @param parentId parent trace id
+		 * @return this
+		 */
+		TraceContext.Builder parentId(String parentId);
+
+		/**
+		 * Sets span id on the trace context.
+		 * @param spanId span id
+		 * @return this
+		 */
+		TraceContext.Builder spanId(String spanId);
+
+		/**
+		 * Sets sampled on the trace context.
+		 * @param sampled if span is sampled
+		 * @return this
+		 */
+		TraceContext.Builder sampled(Boolean sampled);
+
+		/**
+		 * Builds the trace context.
+		 * @return trace context
+		 */
+		TraceContext build();
+
+	}
+
 }

spring-cloud-sleuth-api/src/main/java/org/springframework/cloud/sleuth/Tracer.java

@@ -134,6 +134,12 @@ public interface Tracer extends BaggageManager {
 	 */
 	Span.Builder spanBuilder();
 
+	/**
+	 * Builder for {@link TraceContext}.
+	 * @return a trace context builder
+	 */
+	TraceContext.Builder traceContextBuilder();
+
 	/**
 	 * Allows to customize the current span in scope.
 	 * @return current span customizer

spring-cloud-sleuth-api/src/main/java/org/springframework/cloud/sleuth/WithThreadLocalSpan.java

@@ -0,0 +1,91 @@
+/*
+ * Copyright 2013-2021 the original author or 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
+ *
+ *      https://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 org.springframework.cloud.sleuth;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+
+import org.springframework.lang.Nullable;
+
+/**
+ * Represents a {@link Span} stored in thread local.
+ *
+ * @author Marcin Grzejszczak
+ * @since 3.1.0
+ */
+public interface WithThreadLocalSpan {
+
+	/**
+	 * Logger.
+	 */
+	Log log = LogFactory.getLog(WithThreadLocalSpan.class);
+
+	/**
+	 * Sets the span in thread local scope.
+	 * @param span span to put in thread local
+	 */
+	default void setSpanInScope(Span span) {
+		getThreadLocalSpan().set(span);
+		if (log.isDebugEnabled()) {
+			log.debug("Put span in scope " + span);
+		}
+	}
+
+	/**
+	 * Finishes the thread local span.
+	 * @param error potential error to be stored in span
+	 */
+	default void finishSpan(@Nullable Throwable error) {
+		SpanAndScope spanAndScope = takeSpanFromThreadLocal();
+		if (spanAndScope == null) {
+			return;
+		}
+		Span span = spanAndScope.getSpan();
+		Tracer.SpanInScope scope = spanAndScope.getScope();
+		if (span.isNoop()) {
+			if (log.isDebugEnabled()) {
+				log.debug("Span " + span + " is noop - will stope the scope");
+			}
+			scope.close();
+			return;
+		}
+		if (error != null) { // an error occurred, adding error to span
+			span.error(error);
+		}
+		if (log.isDebugEnabled()) {
+			log.debug("Will finish the span and its corresponding scope " + span);
+		}
+		span.end();
+		scope.close();
+	}
+
+	/**
+	 * Takes a span from thread local and restores the previous one if present.
+	 * @return span from a thread local span
+	 */
+	default SpanAndScope takeSpanFromThreadLocal() {
+		SpanAndScope span = getThreadLocalSpan().get();
+		if (log.isDebugEnabled()) {
+			log.debug("Took span [" + span + "] from thread local");
+		}
+		getThreadLocalSpan().remove();
+		return span;
+	}
+
+	ThreadLocalSpan getThreadLocalSpan();
+
+}