Add convenient method FinishedSpan::getDuration (#265)

Author: quaff

micrometer-tracing-bridges/micrometer-tracing-bridge-brave/src/test/java/io/micrometer/tracing/brave/bridge/BraveFinishedSpanTests.java

@@ -58,6 +58,7 @@ void should_calculate_instant_from_brave_timestamps() {
 
         then(finishedSpan.getStartTimestamp().toEpochMilli()).isEqualTo(TimeUnit.MICROSECONDS.toMillis(startMicros));
         then(finishedSpan.getEndTimestamp().toEpochMilli()).isEqualTo(TimeUnit.MICROSECONDS.toMillis(endMicros));
+        then(finishedSpan.getDuration().toNanos()).isEqualTo(TimeUnit.MICROSECONDS.toNanos(endMicros - startMicros));
     }
 
     @Test

micrometer-tracing-bridges/micrometer-tracing-bridge-otel/src/test/java/io/micrometer/tracing/otel/bridge/OtelFinishedSpanTests.java

@@ -142,6 +142,7 @@ void should_calculate_instant_from_otel_timestamps() {
 
         then(finishedSpan.getStartTimestamp().toEpochMilli()).isEqualTo(TimeUnit.NANOSECONDS.toMillis(startMicros));
         then(finishedSpan.getEndTimestamp().toEpochMilli()).isEqualTo(TimeUnit.NANOSECONDS.toMillis(endMicros));
+        then(finishedSpan.getDuration().toNanos()).isEqualTo(endMicros - startMicros);
     }
 
     @Test

micrometer-tracing/src/main/java/io/micrometer/tracing/exporter/FinishedSpan.java

@@ -15,6 +15,7 @@
  */
 package io.micrometer.tracing.exporter;
 
+import java.time.Duration;
 import java.time.Instant;
 import java.util.Collection;
 import java.util.Collections;
@@ -34,6 +35,7 @@
  *
  * @author OpenZipkin Brave Authors
  * @author Marcin Grzejszczak
+ * @author Yanming Zhou
  * @since 1.0.0
  */
 public interface FinishedSpan {
@@ -60,6 +62,14 @@ public interface FinishedSpan {
      */
     Instant getEndTimestamp();
 
+    /**
+     * @return span's duration
+     * @since 1.2.0
+     */
+    default Duration getDuration() {
+        return Duration.between(getStartTimestamp(), getEndTimestamp());
+    }
+
     /**
      * Sets the tags.
      * @param tags tags to set