Prepare scaffolding for testing x_goog_spanner_request_id

There aren't existent tests for span attributes like `gfe_latency`
and this change begins the process for checking against such, behind
a feature flag.

Author: odeke-em

google-cloud-spanner/src/test/java/com/google/cloud/spanner/XGoogSpannerRequestIdTest.java

@@ -27,6 +27,15 @@
 import io.grpc.ServerCallHandler;
 import io.grpc.ServerInterceptor;
 import io.grpc.Status;
+import io.opentelemetry.api.GlobalOpenTelemetry;
+import io.opentelemetry.api.common.AttributeKey;
+import io.opentelemetry.api.trace.propagation.W3CTraceContextPropagator;
+import io.opentelemetry.context.propagation.ContextPropagators;
+import io.opentelemetry.sdk.OpenTelemetrySdk;
+import io.opentelemetry.sdk.testing.exporter.InMemorySpanExporter;
+import io.opentelemetry.sdk.trace.SdkTracerProvider;
+import io.opentelemetry.sdk.trace.data.SpanData;
+import io.opentelemetry.sdk.trace.export.SimpleSpanProcessor;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
@@ -35,6 +44,8 @@
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.regex.Matcher;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -68,6 +79,13 @@ public static class ServerHeaderEnforcer implements ServerInterceptor {
     private Map<String, CopyOnWriteArrayList<XGoogSpannerRequestId>> streamingResults;
     private List<String> gotValues;
     private Set<String> checkMethods;
+    private static InMemorySpanExporter spanExporter;
+    private static OpenTelemetrySdk openTelemetry;
+
+    // This variable when set to true will allow checking for span
+    // attributes like `x_goog_spanner_request_id`, of which that
+    // requires considerable plumbing in this package.
+    private static final boolean ENABLED_USER_DEFINED_TRACES = false;
 
     ServerHeaderEnforcer(Set<String> checkMethods) {
       this.gotValues = new CopyOnWriteArrayList<String>();
@@ -76,6 +94,38 @@ public static class ServerHeaderEnforcer implements ServerInterceptor {
       this.streamingResults =
           new ConcurrentHashMap<String, CopyOnWriteArrayList<XGoogSpannerRequestId>>();
       this.checkMethods = checkMethods;
+      ServerHeaderEnforcer.setupOpenTelemetry();
+    }
+
+    @BeforeClass
+    public static void setupOpenTelemetry() {
+      SpannerOptions.resetActiveTracingFramework();
+      SpannerOptions.enableOpenTelemetryTraces();
+      GlobalOpenTelemetry.resetForTest();
+
+      spanExporter = InMemorySpanExporter.create();
+
+      SdkTracerProvider tracerProvider =
+          SdkTracerProvider.builder()
+              .addSpanProcessor(SimpleSpanProcessor.create(spanExporter))
+              .build();
+
+      openTelemetry =
+          OpenTelemetrySdk.builder()
+              .setPropagators(ContextPropagators.create(W3CTraceContextPropagator.getInstance()))
+              .setTracerProvider(tracerProvider)
+              .buildAndRegisterGlobal();
+    }
+
+    @AfterClass
+    public static void closeOpenTelemetry() {
+      if (openTelemetry != null) {
+        openTelemetry.close();
+      }
+    }
+
+    public List<SpanData> getFinishedSpans() {
+      return this.spanExporter.getFinishedSpanItems();
     }
 
     @Override
@@ -108,6 +158,15 @@ public <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(
       // Firstly assert and validate that at least we've got a requestId.
       Matcher m = XGoogSpannerRequestId.REGEX.matcher(gotReqIdStr);
       assertTrue(m.matches());
+      if (ENABLED_USER_DEFINED_TRACES) {
+        List<SpanData> spans = this.getFinishedSpans();
+        assertNotNull(spans);
+        SpanData span = spans.get(0);
+        String gotReqIdFromSpanAttribute =
+            span.getAttributes().get(AttributeKey.stringKey("x_goog_spanner_request_id"));
+        assertNotNull(gotReqIdFromSpanAttribute);
+        assertEquals(gotReqIdStr, gotReqIdFromSpanAttribute);
+      }
 
       XGoogSpannerRequestId reqId = XGoogSpannerRequestId.of(gotReqIdStr);
       if (!saver.containsKey(methodName)) {