Add a new log method to span that allows for setting the time (#501)

* Add a new log method that allows for setting the time
* Clarify with time unit
* Simple test and spelling correction
* Consolidate "logic" and add a test for the other log method

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

@@ -340,7 +340,15 @@ public class Span implements SpanContext {
 	 * Add an {@link Log#event event} to the timeline associated with this span.
 	 */
 	public void logEvent(String event) {
-		this.logs.add(new Log(System.currentTimeMillis(), event));
+		logEvent(System.currentTimeMillis(), event);
+	}
+
+	/**
+	 * Add a {@link Log#event event} to a specific point (a timestamp in milliseconds) in the timeline
+	 * associated with this span.
+	 */
+	public void logEvent(long timestampMilliseconds, String event) {
+		this.logs.add(new Log(timestampMilliseconds, event));
 	}
 
 	/**

spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/SpanTests.java

@@ -166,6 +166,19 @@ public class SpanTests {
 				.isEqualTo(span.logs());
 	}
 
+	@Test public void can_log_with_generated_timestamp() throws IOException {
+		span.logEvent("event1");
+
+		long beforeLog = System.currentTimeMillis();
+		assertThat(span.logs().get(0).getTimestamp()).isGreaterThanOrEqualTo(beforeLog);
+	}
+
+	@Test public void can_log_with_specified_timestamp() throws IOException {
+		span.logEvent(1L, "event1");
+
+		then(span.logs().get(0).getTimestamp()).isEqualTo(1L);
+	}
+
 	@Test public void should_properly_serialize_tags() throws IOException {
 		span.tag("calculatedTax", "100");