Fixes from review.

exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/MetricAdapter.java

@@ -221,6 +221,7 @@ private static void addHistogramSamples(
     }
   }
 
+  @Nullable
   private static Exemplar lastExemplarOrNull(Collection<Exemplar> exemplars) {
     Exemplar result = null;
     for (Exemplar e : exemplars) {
@@ -229,6 +230,7 @@ private static Exemplar lastExemplarOrNull(Collection<Exemplar> exemplars) {
     return result;
   }
 
+  @Nullable
   private static Exemplar filterExemplars(Collection<Exemplar> exemplars, double min, double max) {
     Exemplar result = null;
     for (Exemplar e : exemplars) {

exporters/prometheus/src/test/java/io/opentelemetry/exporter/prometheus/MetricAdapterTest.java

@@ -433,7 +433,10 @@ public String fallbackToStringOf(Object object) {
           return sb.toString();
         }
       }
-      return object.toString();
+      if (object != null) {
+        return super.fallbackToStringOf(object);
+      }
+      return "null";
     }
     /** Convert an exemplar into a human readable string. */
     private static String exemplarToString(Exemplar exemplar) {

sdk/metrics-testing/src/test/java/io/opentelemetry/sdk/testing/assertj/metrics/MetricAssertionsTest.java

@@ -23,7 +23,6 @@
 import io.opentelemetry.sdk.metrics.data.LongSumData;
 import io.opentelemetry.sdk.metrics.data.MetricData;
 import io.opentelemetry.sdk.resources.Resource;
-import java.util.Arrays;
 import java.util.Collections;
 import org.junit.jupiter.api.Test;
 
@@ -100,7 +99,8 @@ public class MetricAssertionsTest {
       DoublePointData.create(1, 2, Attributes.empty(), 3.0, Collections.emptyList());
 
   private static final DoublePointData DOUBLE_POINT_DATA_WITH_EXEMPLAR =
-      DoublePointData.create(1, 2, Attributes.empty(), 3.0, Arrays.asList(DOUBLE_EXEMPLAR));
+      DoublePointData.create(
+          1, 2, Attributes.empty(), 3.0, Collections.singletonList(DOUBLE_EXEMPLAR));
 
   private static final MetricData LONG_GAUGE_METRIC =
       MetricData.createLongGauge(
@@ -146,7 +146,7 @@ public class MetricAssertionsTest {
       LongPointData.create(1, 2, Attributes.empty(), 3, Collections.emptyList());
 
   private static final LongPointData LONG_POINT_DATA_WITH_EXEMPLAR =
-      LongPointData.create(1, 2, Attributes.empty(), 3, Arrays.asList(LONG_EXEMPLAR));
+      LongPointData.create(1, 2, Attributes.empty(), 3, Collections.singletonList(LONG_EXEMPLAR));
 
   @Test
   void metric_passing() {

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/data/DoubleHistogramPointData.java

@@ -8,7 +8,6 @@
 import com.google.auto.value.AutoValue;
 import io.opentelemetry.api.common.Attributes;
 import java.util.ArrayList;
-import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 import javax.annotation.concurrent.Immutable;
@@ -53,7 +52,7 @@ public static DoubleHistogramPointData create(
       double sum,
       List<Double> boundaries,
       List<Long> counts,
-      Collection<Exemplar> exemplars) {
+      List<Exemplar> exemplars) {
     if (counts.size() != boundaries.size() + 1) {
       throw new IllegalArgumentException(
           "invalid counts: size should be "

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/data/DoublePointData.java

@@ -7,8 +7,8 @@
 
 import com.google.auto.value.AutoValue;
 import io.opentelemetry.api.common.Attributes;
-import java.util.Collection;
 import java.util.Collections;
+import java.util.List;
 import javax.annotation.concurrent.Immutable;
 
 /**
@@ -50,7 +50,7 @@ public static DoublePointData create(
       long epochNanos,
       Attributes attributes,
       double value,
-      Collection<Exemplar> exemplars) {
+      List<Exemplar> exemplars) {
     return new AutoValue_DoublePointData(startEpochNanos, epochNanos, attributes, exemplars, value);
   }
 

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/data/Exemplar.java

@@ -6,6 +6,7 @@
 package io.opentelemetry.sdk.metrics.data;
 
 import io.opentelemetry.api.common.Attributes;
+import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 
 /**
@@ -29,16 +30,18 @@ public interface Exemplar {
   /**
    * (Optional) Span ID of the exemplar trace.
    *
-   * <p>Span ID may be missing if the measurement is not recorded inside a trace or the trace was
-   * not sampled.
+   * <p>Span ID may be {@code null} if the measurement is not recorded inside a trace or the trace
+   * was not sampled.
    */
+  @Nullable
   String getSpanId();
   /**
    * (Optional) Trace ID of the exemplar trace.
    *
-   * <p>Trace ID may be missing if the measurement is not recorded inside a trace or if the trace is
-   * not sampled.
+   * <p>Trace ID may be {@code null} if the measurement is not recorded inside a trace or if the
+   * trace is not sampled.
    */
+  @Nullable
   String getTraceId();
 
   /**

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/data/LongPointData.java

@@ -7,8 +7,8 @@
 
 import com.google.auto.value.AutoValue;
 import io.opentelemetry.api.common.Attributes;
-import java.util.Collection;
 import java.util.Collections;
+import java.util.List;
 import javax.annotation.concurrent.Immutable;
 
 /**
@@ -61,7 +61,7 @@ public static LongPointData create(
       long epochNanos,
       Attributes attributes,
       long value,
-      Collection<Exemplar> exemplars) {
+      List<Exemplar> exemplars) {
     return new AutoValue_LongPointData(startEpochNanos, epochNanos, attributes, exemplars, value);
   }
 }

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/data/PointData.java

@@ -6,7 +6,7 @@
 package io.opentelemetry.sdk.metrics.data;
 
 import io.opentelemetry.api.common.Attributes;
-import java.util.Collection;
+import java.util.List;
 import javax.annotation.concurrent.Immutable;
 
 /**
@@ -40,5 +40,5 @@ public interface PointData {
    */
   Attributes getAttributes();
   /** List of exemplars collected from measurements that were used to form the data point. */
-  Collection<Exemplar> getExemplars();
+  List<Exemplar> getExemplars();
 }