Added ForceFlush Implementation (#885)

### Description:
Passed the forceflush function from SdkMeteringProvider to the
AwsSpanMetricProcessor to forceFlush remaining metrics on shutdown to
the cwAgent/Collector.

### Tesing:
Increased the metricExporter interval and the BatchSpanProcessor delay
to 10 minutes using:
```
OTEL_METRIC_EXPORT_INTERVAL=600000 \
OTEL_BSP_SCHEDULE_DELAY=600000 \
```
Without the force flush change, exiting the sample app only flushed the
traces without the metrics. With the forceFlush change, both traces and
metrics were flushed to the collector.


By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.

---------

Co-authored-by: Mohamed Asaker <[email protected]>

awsagentprovider/src/main/java/software/amazon/opentelemetry/javaagent/providers/AwsApplicationSignalsCustomizerProvider.java

@@ -17,7 +17,6 @@
 
 import io.opentelemetry.api.common.Attributes;
 import io.opentelemetry.api.common.AttributesBuilder;
-import io.opentelemetry.api.metrics.MeterProvider;
 import io.opentelemetry.contrib.awsxray.AlwaysRecordSampler;
 import io.opentelemetry.contrib.awsxray.ResourceHolder;
 import io.opentelemetry.exporter.otlp.http.metrics.OtlpHttpMetricExporter;
@@ -164,14 +163,16 @@ private SdkTracerProviderBuilder customizeTracerProviderBuilder(
       MetricReader metricReader =
           PeriodicMetricReader.builder(metricsExporter).setInterval(exportInterval).build();
 
-      MeterProvider meterProvider =
+      SdkMeterProvider meterProvider =
           SdkMeterProvider.builder()
               .setResource(ResourceHolder.getResource())
               .registerMetricReader(metricReader)
               .build();
+
       // Construct and set application signals metrics processor
       SpanProcessor spanMetricsProcessor =
-          AwsSpanMetricsProcessorBuilder.create(meterProvider, ResourceHolder.getResource())
+          AwsSpanMetricsProcessorBuilder.create(
+                  meterProvider, ResourceHolder.getResource(), meterProvider::forceFlush)
               .build();
       tracerProviderBuilder.addSpanProcessor(spanMetricsProcessor);
     }

awsagentprovider/src/main/java/software/amazon/opentelemetry/javaagent/providers/AwsSpanMetricsProcessor.java

@@ -22,12 +22,14 @@
 import io.opentelemetry.api.metrics.LongHistogram;
 import io.opentelemetry.api.trace.StatusCode;
 import io.opentelemetry.context.Context;
+import io.opentelemetry.sdk.common.CompletableResultCode;
 import io.opentelemetry.sdk.resources.Resource;
 import io.opentelemetry.sdk.trace.ReadWriteSpan;
 import io.opentelemetry.sdk.trace.ReadableSpan;
 import io.opentelemetry.sdk.trace.SpanProcessor;
 import io.opentelemetry.sdk.trace.data.SpanData;
 import java.util.Map;
+import java.util.function.Supplier;
 import javax.annotation.concurrent.Immutable;
 
 /**
@@ -64,29 +66,38 @@ public final class AwsSpanMetricsProcessor implements SpanProcessor {
 
   private final MetricAttributeGenerator generator;
   private final Resource resource;
+  private final Supplier<CompletableResultCode> forceFlushAction;
 
   /** Use {@link AwsSpanMetricsProcessorBuilder} to construct this processor. */
   static AwsSpanMetricsProcessor create(
       LongHistogram errorHistogram,
       LongHistogram faultHistogram,
       DoubleHistogram latencyHistogram,
       MetricAttributeGenerator generator,
-      Resource resource) {
+      Resource resource,
+      Supplier<CompletableResultCode> forceFlushAction) {
     return new AwsSpanMetricsProcessor(
-        errorHistogram, faultHistogram, latencyHistogram, generator, resource);
+        errorHistogram, faultHistogram, latencyHistogram, generator, resource, forceFlushAction);
   }
 
   private AwsSpanMetricsProcessor(
       LongHistogram errorHistogram,
       LongHistogram faultHistogram,
       DoubleHistogram latencyHistogram,
       MetricAttributeGenerator generator,
-      Resource resource) {
+      Resource resource,
+      Supplier<CompletableResultCode> forceFlushAction) {
     this.errorHistogram = errorHistogram;
     this.faultHistogram = faultHistogram;
     this.latencyHistogram = latencyHistogram;
     this.generator = generator;
     this.resource = resource;
+    this.forceFlushAction = forceFlushAction;
+  }
+
+  @Override
+  public CompletableResultCode forceFlush() {
+    return forceFlushAction.get();
   }
 
   @Override

awsagentprovider/src/main/java/software/amazon/opentelemetry/javaagent/providers/AwsSpanMetricsProcessorBuilder.java

@@ -22,7 +22,9 @@
 import io.opentelemetry.api.metrics.LongHistogram;
 import io.opentelemetry.api.metrics.Meter;
 import io.opentelemetry.api.metrics.MeterProvider;
+import io.opentelemetry.sdk.common.CompletableResultCode;
 import io.opentelemetry.sdk.resources.Resource;
+import java.util.function.Supplier;
 
 /** A builder for {@link AwsSpanMetricsProcessor} */
 public final class AwsSpanMetricsProcessorBuilder {
@@ -42,18 +44,29 @@ public final class AwsSpanMetricsProcessorBuilder {
   private final MeterProvider meterProvider;
   private final Resource resource;
 
+  // ForceFlush action provided from {@link SdkMeterProvider#forceFlush()} so that when the
+  // application exits The spanMetricProcessor calls the meterProvder.forceFlush to flush
+  // any remaining metrics before shutdown
+  private final Supplier<CompletableResultCode> forceFlushAction;
+
   // Optional builder elements
   private MetricAttributeGenerator generator = DEFAULT_GENERATOR;
   private String scopeName = DEFAULT_SCOPE_NAME;
 
   public static AwsSpanMetricsProcessorBuilder create(
-      MeterProvider meterProvider, Resource resource) {
-    return new AwsSpanMetricsProcessorBuilder(meterProvider, resource);
+      MeterProvider meterProvider,
+      Resource resource,
+      Supplier<CompletableResultCode> forceFlushAction) {
+    return new AwsSpanMetricsProcessorBuilder(meterProvider, resource, forceFlushAction);
   }
 
-  private AwsSpanMetricsProcessorBuilder(MeterProvider meterProvider, Resource resource) {
+  private AwsSpanMetricsProcessorBuilder(
+      MeterProvider meterProvider,
+      Resource resource,
+      Supplier<CompletableResultCode> forceFlushAction) {
     this.meterProvider = meterProvider;
     this.resource = resource;
+    this.forceFlushAction = forceFlushAction;
   }
 
   /**
@@ -86,6 +99,6 @@ public AwsSpanMetricsProcessor build() {
         meter.histogramBuilder(LATENCY).setUnit(LATENCY_UNITS).build();
 
     return AwsSpanMetricsProcessor.create(
-        errorHistogram, faultHistogram, latencyHistogram, generator, resource);
+        errorHistogram, faultHistogram, latencyHistogram, generator, resource, forceFlushAction);
   }
 }

awsagentprovider/src/test/java/software/amazon/opentelemetry/javaagent/providers/AwsSpanMetricsProcessorTest.java

@@ -77,6 +77,12 @@ private enum ExpectedStatusMetric {
   private MetricAttributeGenerator generatorMock;
   private AwsSpanMetricsProcessor awsSpanMetricsProcessor;
 
+  // Mock forceFlush function that returns success when invoked similar
+  // to the default implementation of forceFlush.
+  private CompletableResultCode forceFlushAction() {
+    return CompletableResultCode.ofSuccess();
+  }
+
   @BeforeEach
   public void setUpMocks() {
     errorHistogramMock = mock(LongHistogram.class);
@@ -90,7 +96,8 @@ public void setUpMocks() {
             faultHistogramMock,
             latencyHistogramMock,
             generatorMock,
-            testResource);
+            testResource,
+            this::forceFlushAction);
   }
 
   @Test