Adds the ability to configure things needed by stackdriver

We will eventually need to change the encoder to use their format. Also,
stackdriver does not support shared spans.

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

@@ -34,6 +34,9 @@ public class SleuthProperties {
 	/** When true, generate 128-bit trace IDs instead of 64-bit ones. */
 	private boolean traceId128 = false;
 
+	/** True means the tracing system supports sharing a span ID between a client and server. */
+	private boolean supportsJoin = true;
+
 	/**
 	 * List of baggage key names that should be propagated out of process.
 	 * These keys will be prefixed with `baggage` before the actual key.
@@ -70,6 +73,14 @@ public class SleuthProperties {
 		this.traceId128 = traceId128;
 	}
 
+	public boolean isSupportsJoin() {
+		return this.supportsJoin;
+	}
+
+	public void setSupportsJoin(boolean supportsJoin) {
+		this.supportsJoin = supportsJoin;
+	}
+
 	public List<String> getBaggageKeys() {
 		return this.baggageKeys;
 	}

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

@@ -74,6 +74,7 @@ public class TraceAutoConfiguration {
 				.currentTraceContext(currentTraceContext)
 				.spanReporter(adjustedReporter(reporter))
 				.traceId128Bit(sleuthProperties.isTraceId128())
+				.supportsJoin(sleuthProperties.isSupportsJoin())
 				.build();
 	}