samples: add GFELatency metrics to TracingSample (#1559)

* feat: Adding GFELatency metrics to TracingSample

* Deleted metrics sample

* Updating README and changing deprecated method in TracingSample

* 🦉 Updates from OwlBot

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Thiago Nunes <thiagotnunes@google.com>

samples/snippets/src/main/java/com/example/spanner/TracingSample.java

@@ -22,6 +22,7 @@
 import com.google.cloud.spanner.Spanner;
 import com.google.cloud.spanner.SpannerOptions;
 import com.google.cloud.spanner.Statement;
+import com.google.cloud.spanner.spi.v1.SpannerRpcViews;
 import io.opencensus.common.Scope;
 import io.opencensus.contrib.grpc.metrics.RpcViews;
 import io.opencensus.contrib.zpages.ZPageHandlers;
@@ -54,7 +55,9 @@ public static void main(String[] args) throws Exception {
 
     // Installs an exporter for stack driver stats.
     StackdriverStatsExporter.createAndRegister();
-    RpcViews.registerAllCumulativeViews();
+    RpcViews.registerAllGrpcViews();
+    // Capture GFE Latency and GFE Header missing count.
+    SpannerRpcViews.registerGfeLatencyAndHeaderMissingCountViews();
 
     // Name of your instance & database.
     String instanceId = args[0];