Wire exemplar filter config into autoconfigure module.

Author: jsuereth

sdk-extensions/autoconfigure/README.md

@@ -191,6 +191,12 @@ These properties can be used to control the maximum size of recordings per span.
 | otel.span.event.count.limit            | OTEL_SPAN_EVENT_COUNT_LIMIT            | The maximum number of events per span. Default is `128`.               |
 | otel.span.link.count.limit             | OTEL_SPAN_LINK_COUNT_LIMIT             | The maximum number of links per span. Default is `128`                 |
 
+## Exemplars
+
+| System property          | Environment variable     | Description                                                                       |
+|--------------------------|--------------------------|-----------------------------------------------------------------------------------|
+| otel.metrics.exemplar.filter | OTEL_METRICS_EXEMPLAR_FILTER | The filter for exemplar sampling.  Can be `NONE`, `ALL` or `WITH_SAMPLED_TRACE`. Default is `WITH_SAMPLED_TRACE`.|
+
 ## Interval metric reader
 
 | System property          | Environment variable     | Description                                                                       |

sdk-extensions/autoconfigure/src/main/java/io/opentelemetry/sdk/autoconfigure/OpenTelemetrySdkAutoConfiguration.java

@@ -12,6 +12,8 @@
 import io.opentelemetry.sdk.autoconfigure.spi.metrics.SdkMeterProviderConfigurer;
 import io.opentelemetry.sdk.metrics.SdkMeterProvider;
 import io.opentelemetry.sdk.metrics.SdkMeterProviderBuilder;
+import io.opentelemetry.sdk.metrics.exemplar.ExemplarFilter;
+import io.opentelemetry.sdk.metrics.exemplar.ExemplarSampler;
 import io.opentelemetry.sdk.resources.Resource;
 import io.opentelemetry.sdk.trace.SdkTracerProvider;
 import java.util.ServiceLoader;
@@ -90,6 +92,27 @@ private static void configureMeterProvider(Resource resource, ConfigProperties c
 
     SdkMeterProviderBuilder meterProviderBuilder = SdkMeterProvider.builder().setResource(resource);
 
+    // Configure default exemplar filters.
+    String exemplarFilter = config.getString("otel.metrics.exemplar.filter");
+    if (exemplarFilter == null) {
+      exemplarFilter = "WITH_SAMPLED_TRACE";
+    }
+    switch (exemplarFilter) {
+      case "NONE":
+        meterProviderBuilder.setExemplarSampler(
+            ExemplarSampler.builder().setFilter(ExemplarFilter.neverSample()).build());
+        break;
+      case "ALL":
+        meterProviderBuilder.setExemplarSampler(
+            ExemplarSampler.builder().setFilter(ExemplarFilter.alwaysSample()).build());
+        break;
+      case "WITH_SAMPLED_TRACE":
+      default:
+        meterProviderBuilder.setExemplarSampler(
+            ExemplarSampler.builder().setFilter(ExemplarFilter.sampleWithTraces()).build());
+        break;
+    }
+
     for (SdkMeterProviderConfigurer configurer :
         ServiceLoader.load(SdkMeterProviderConfigurer.class)) {
       configurer.configure(meterProviderBuilder, config);