Support explicit Span finish times

opentracing-api/src/main/java/io/opentracing/Span.java

@@ -22,18 +22,34 @@ public interface Span extends AutoCloseable {
     /**
      * Retrieve the associated SpanContext.
      *
+     * This may be called at any time, including after calls to finish().
+     *
      * @return the SpanContext that encapsulates Span state that should propagate across process boundaries.
      */
     SpanContext context();
 
     /**
-     * Sets the end timestamp and records the span.
+     * Sets the end timestamp to now and records the span.
+     *
+     * <p>With the exception of calls to Span.context(), this should be the last call made to the span instance, and to
+     * do otherwise leads to undefined behavior.
      *
-     * <p>This should be the last call made to any span instance, and to do otherwise leads to
-     * undefined behavior.
+     * @see Span#context()
      */
     void finish();
 
+    /**
+     * Sets an explicit end timestamp and records the span.
+     *
+     * <p>With the exception of calls to Span.context(), this should be the last call made to the span instance, and to
+     * do otherwise leads to undefined behavior.
+     *
+     * @param finishMicros an explicit finish time, in microseconds since the epoch
+     *
+     * @see Span#context()
+     */
+    void finish(long finishMicros);
+
     void close();
 
     /**

opentracing-impl-java8/src/main/java/io/opentracing/AbstractSpan.java

@@ -43,6 +43,13 @@ public void finish() {
         assert null == duration;
         duration = Duration.between(start, Instant.now());
     }
+    @Override
+    public void finish(long finishMicros) {
+        long finishEpochSeconds = TimeUnit.MICROSECONDS.toSeconds(finishMicros);
+        long nanos = TimeUnit.MICROSECONDS.toNanos(finishMicros) - TimeUnit.SECONDS.toNanos(finishEpochSeconds);
+        assert null == duration;
+        duration = Duration.between(start, Instant.ofEpochSecond(finishEpochSeconds, nanos));
+    }
 
     @Override
     public void close() {

opentracing-impl/src/main/java/io/opentracing/NoopSpan.java

@@ -26,6 +26,9 @@ private NoopSpan() {}
     @Override
     public void finish() {}
 
+    @Override
+    public void finish(long finishMicros) {}
+
     @Override
     public void close() {
         finish();