Treat gauge.update() parameter as new value, not delta

jaeger-core/src/main/java/io/jaegertracing/metrics/InMemoryMetricsFactory.java

@@ -62,7 +62,7 @@ public Gauge createGauge(final String name, final Map<String, String> tags) {
     return new Gauge() {
       @Override
       public void update(long amount) {
-        value.addAndGet(amount);
+        value.getAndSet(amount);
       }
     };
   }

jaeger-core/src/test/java/io/jaegertracing/metrics/InMemoryMetricsFactoryTest.java

@@ -85,16 +85,18 @@ public void metricNameIsUsedForGauge() {
   }
 
   @Test
-  public void gaugeValueIsIncreased() {
+  public void gaugeValueIsUpdated() {
     Map<String, String> tags = Collections.singletonMap("foo", "bar");
 
     InMemoryMetricsFactory inMemoryMetricsFactory = new InMemoryMetricsFactory();
     Gauge gauge = inMemoryMetricsFactory.createGauge("thegauge", tags);
     assertEquals(0, inMemoryMetricsFactory.getGauge("thegauge", tags));
 
     gauge.update(1);
-
     assertEquals(1, inMemoryMetricsFactory.getGauge("thegauge", tags));
+
+    gauge.update(2);
+    assertEquals(2, inMemoryMetricsFactory.getGauge("thegauge", tags));
   }
 
   @Test