[#159] Tagging hystrix events

when a hystrix command was explicitly created we can tag the following values
    * command key
    * command group
    * thread pool key

    fixes #159

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/Span.java

@@ -25,6 +25,7 @@ import java.util.List;
 import java.util.Map;
 
 import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
 
 /**
  * Class for gathering and reporting statistics about a block of execution.
@@ -148,10 +149,13 @@ public class Span {
 	}
 
 	/**
-	 * Add a tag or data annotation associated with this span
+	 * Add a tag or data annotation associated with this span. The tag will be
+	 * added only if it has a value.
 	 */
 	public void tag(String key, String value) {
-		this.tags.put(key, value);
+		if (StringUtils.hasText(value)) {
+			this.tags.put(key, value);
+		}
 	}
 
 	/**

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/instrument/TraceKeys.java

@@ -314,10 +314,35 @@ public class TraceKeys {
 		private String prefix = "";
 
 		/**
-		 * Name of the command key
+		 * Name of the command key. Describes the name for the given command.
+		 * A key to represent a {@link com.netflix.hystrix.HystrixCommand} for
+		 * monitoring, circuit-breakers, metrics publishing, caching and other such uses.
+		 *
+		 * @see com.netflix.hystrix.HystrixCommandKey
 		 */
 		private String commandKey = "commandKey";
 
+		/**
+		 * Name of the command group. Hystrix uses the command group key to group
+		 * together commands such as for reporting, alerting, dashboards,
+		 * or team/library ownership.
+		 *
+		 * @see com.netflix.hystrix.HystrixCommandGroupKey
+		 */
+		private String commandGroup = "commandGroup";
+
+		/**
+		 * Name of the thread pool key. The thread-pool key represents a {@link com.netflix.hystrix.HystrixThreadPool}
+		 * for monitoring, metrics publishing, caching, and other such uses. A {@link com.netflix.hystrix.HystrixCommand}
+		 * is associated with a single {@link com.netflix.hystrix.HystrixThreadPool} as
+		 * retrieved by the {@link com.netflix.hystrix.HystrixThreadPoolKey} injected into it,
+		 * or it defaults to one created using the {@link com.netflix.hystrix.HystrixCommandGroupKey}
+		 * it is created with.
+		 *
+		 * @see com.netflix.hystrix.HystrixThreadPoolKey
+		 */
+		private String threadPoolKey = "threadPoolKey";
+
 		public String getPrefix() {
 			return this.prefix;
 		}
@@ -326,6 +351,14 @@ public class TraceKeys {
 			return this.commandKey;
 		}
 
+		public String getCommandGroup() {
+			return this.commandGroup;
+		}
+
+		public String getThreadPoolKey() {
+			return this.threadPoolKey;
+		}
+
 		public void setPrefix(String prefix) {
 			this.prefix = prefix;
 		}
@@ -334,6 +367,14 @@ public class TraceKeys {
 			this.commandKey = commandKey;
 		}
 
+		public void setCommandGroup(String commandGroup) {
+			this.commandGroup = commandGroup;
+		}
+
+		public void setThreadPoolKey(String threadPoolKey) {
+			this.threadPoolKey = threadPoolKey;
+		}
+
 	}
 
 	/**
@@ -348,17 +389,23 @@ public class TraceKeys {
 
 		/**
 		 * Name of the thread that executed the async method
+		 *
+		 * @see org.springframework.scheduling.annotation.Async
 		 */
 		private String threadNameKey = "thread";
 
 		/**
 		 * Simple name of the class with a method annotated with {@code @Async}
 		 * from which the asynchronous process started
+		 *
+		 * @see org.springframework.scheduling.annotation.Async
 		 */
 		private String classNameKey = "class";
 
 		/**
 		 * Name of the method annotated with {@code @Async}
+		 *
+		 * @see org.springframework.scheduling.annotation.Async
 		 */
 		private String methodNameKey = "method";
 

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/instrument/hystrix/TraceCommand.java

@@ -18,9 +18,9 @@ package org.springframework.cloud.sleuth.instrument.hystrix;
 
 import org.springframework.cloud.sleuth.Span;
 import org.springframework.cloud.sleuth.Tracer;
+import org.springframework.cloud.sleuth.instrument.TraceKeys;
 
 import com.netflix.hystrix.HystrixCommand;
-import org.springframework.cloud.sleuth.instrument.TraceKeys;
 
 /**
  * Abstraction over {@code HystrixCommand} that wraps command execution with Trace setting
@@ -54,6 +54,10 @@ public abstract class TraceCommand<R> extends HystrixCommand<R> {
 		this.tracer.addTag(Span.SPAN_LOCAL_COMPONENT_TAG_NAME, HYSTRIX_COMPONENT);
 		this.tracer.addTag(this.traceKeys.getHystrix().getPrefix() +
 				this.traceKeys.getHystrix().getCommandKey(), commandKeyName);
+		this.tracer.addTag(this.traceKeys.getHystrix().getPrefix() +
+				this.traceKeys.getHystrix().getCommandGroup(), getCommandGroup().name());
+		this.tracer.addTag(this.traceKeys.getHystrix().getPrefix() +
+				this.traceKeys.getHystrix().getThreadPoolKey(), getThreadPoolKey().name());
 		try {
 			return doRun();
 		}

spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/hystrix/TraceCommandTests.java

@@ -2,10 +2,6 @@ package org.springframework.cloud.sleuth.instrument.hystrix;
 
 import java.util.Random;
 
-import com.netflix.hystrix.HystrixCommandKey;
-import com.netflix.hystrix.HystrixCommandProperties;
-import com.netflix.hystrix.HystrixThreadPoolProperties;
-import com.netflix.hystrix.strategy.HystrixPlugins;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
@@ -19,6 +15,11 @@ import org.springframework.cloud.sleuth.trace.DefaultTracer;
 import org.springframework.cloud.sleuth.trace.TestSpanContextHolder;
 import org.springframework.context.ApplicationEventPublisher;
 
+import com.netflix.hystrix.HystrixCommandKey;
+import com.netflix.hystrix.HystrixCommandProperties;
+import com.netflix.hystrix.HystrixThreadPoolProperties;
+import com.netflix.hystrix.strategy.HystrixPlugins;
+
 import static com.netflix.hystrix.HystrixCommand.Setter.withGroupKey;
 import static com.netflix.hystrix.HystrixCommandGroupKey.Factory.asKey;
 import static org.springframework.cloud.sleuth.assertions.SleuthAssertions.then;
@@ -70,8 +71,12 @@ public class TraceCommandTests {
 
 		Span spanFromCommand = whenCommandIsExecuted(command);
 
-		then(spanFromCommand).as("Span from the Hystrix Thread").isNotNull();
-		then(spanFromCommand.getTraceId()).isEqualTo(EXPECTED_TRACE_ID);
+		then(spanFromCommand).as("Span from the Hystrix Thread")
+				.isNotNull()
+				.hasTraceIdEqualTo(EXPECTED_TRACE_ID)
+				.hasATag("commandKey", "traceCommandKey")
+				.hasATag("commandGroup", "group")
+				.hasATag("threadPoolKey", "group");
 	}
 
 	private Span givenATraceIsPresentInTheCurrentThread() {