Prefers the word event when adding to a Span's timeline

Span.logs are really timestamped events. This clarifies the naming and
documentation around that.

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

@@ -25,12 +25,25 @@ import lombok.RequiredArgsConstructor;
 @Data
 @RequiredArgsConstructor
 public class Log {
-	private final long time;
-	private final String msg;
+	/**
+	 * The epoch timestamp of the log record; often set via {@link System#currentTimeMillis()}.
+	 */
+	private final long timestamp;
+
+	/**
+	 * Event (if not null) should be the stable name of some notable moment in the lifetime of a Span.
+	 * For instance, a Span representing a browser page load might add an Event for each of the
+	 * Performance.timing moments here: https://developer.mozilla.org/en-US/docs/Web/API/PerformanceTiming
+	 *
+	 * <p>While it is not a formal requirement, Event strings will be most useful if they are *not*
+	 * unique; rather, tracing systems should be able to use them to understand how two similar Spans
+	 * relate from an internal timing perspective.
+	 */
+	private final String event;
 
 	@SuppressWarnings("unused")
 	private Log() {
-		this.time = 0;
-		this.msg = null;
+		this.timestamp = 0;
+		this.event = null;
 	}
 }

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

@@ -170,11 +170,10 @@ public class Span {
 	}
 
 	/**
-	 * Add a log or timeline annotation associated with this span
+	 * Add an {@link Log#event event} to the timeline associated with this span.
 	 */
-	public void log(String msg) {
-		this.logs.add(new Log(System.currentTimeMillis(),
-				msg));
+	public void logEvent(String event) {
+		this.logs.add(new Log(System.currentTimeMillis(), event));
 	}
 
 	/**
@@ -188,7 +187,7 @@ public class Span {
 	}
 
 	/**
-	 * Get any logs or annotations (read only)
+	 * Get any timestamped events (read only)
 	 * <p/>
 	 * <p/>
 	 * Will never be null.

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

@@ -48,7 +48,7 @@ public class JsonLogSpanListenerTests {
 				.end(10)
 				.build();
 		span.tag("myKey", "myVal");
-		span.log("myTimelineAnnotation");
+		span.logEvent("myTimelineAnnotation");
 		listener.stop(new SpanReleasedEvent(this, span));
 
 		String output = this.output.toString().trim();