fix: Shutdown Stackdriver MetricServiceClient properly (#2091)

Using the default configuration of StackdriverStatsConfiguration, a
MetricServiceClient is initialized by the StackdriverStatsExporter.

This client was never closed, this will be done now when the
.unregister() method is called on the exporter.

If a custom MetricServiceStub is given by the user, it will *not* be
closed as the user should be in charge of it.

CHANGELOG.md

@@ -1,5 +1,7 @@
 ## Unreleased
 
+- fix: Shutdown `MetricServiceClient` properly on `StackdriverStatsExporter.unregister()` (#2007)
+
 ## 0.28.3 - 2021-01-12
 
 - fix: Return public access to unsafe `ContextUtils` api.  Remove bincompat issue from 0.27.1. (#2072)

exporters/stats/stackdriver/src/main/java/io/opencensus/exporter/stats/stackdriver/StackdriverStatsExporter.java

@@ -77,6 +77,10 @@ public final class StackdriverStatsExporter {
   @Nullable
   private static StackdriverStatsExporter instance = null;
 
+  @GuardedBy("monitor")
+  @Nullable
+  private static MetricServiceClient metricServiceClient = null;
+
   private static final String EXPORTER_SPAN_NAME = "ExportMetricsToStackdriver";
 
   // See io.grpc.internal.GrpcUtil.USER_AGENT_KEY
@@ -392,10 +396,13 @@ private static void createInternal(
       throws IOException {
     synchronized (monitor) {
       checkState(instance == null, "Stackdriver stats exporter is already created.");
-      MetricServiceClient client =
-          stub == null
-              ? createMetricServiceClient(credentials, deadline)
-              : MetricServiceClient.create(stub);
+      final MetricServiceClient client;
+      if (stub == null) {
+        metricServiceClient = createMetricServiceClient(credentials, deadline);
+        client = metricServiceClient;
+      } else {
+        client = MetricServiceClient.create(stub);
+      }
       instance =
           new StackdriverStatsExporter(
               projectId,
@@ -445,6 +452,10 @@ public static void unregister() {
         instance.intervalMetricReader.stop();
       }
       instance = null;
+      if (metricServiceClient != null) {
+        metricServiceClient.close();
+        metricServiceClient = null;
+      }
     }
   }
 }