test: switch tracing end-to-end tests to nightly jobs. (#1812)

* test: switch tracing end-to-end tests to nightly jobs.

* env vars set in github should begin with `GITHUB_`

* testing.

* remove print stmt and fix up.

Author: ehsannas

java-firestore/.kokoro/nightly/common.cfg

@@ -23,3 +23,8 @@ env_vars: {
   key: "JOB_TYPE"
   value: "test"
 }
+
+env_vars: {
+  key: "GITHUB_ENV_VAR_KOKORO_JOB_TYPE"
+  value: "nightly"
+}

java-firestore/.kokoro/presubmit/common.cfg

@@ -24,6 +24,11 @@ env_vars: {
   value: "test"
 }
 
+env_vars: {
+  key: "GITHUB_ENV_VAR_KOKORO_JOB_TYPE"
+  value: "presubmit"
+}
+
 before_action {
   fetch_keystore {
     keystore_resource {

java-firestore/google-cloud-firestore/src/test/java/com/google/cloud/firestore/it/ITE2ETracingTest.java

@@ -40,6 +40,7 @@
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
+import static org.junit.Assume.assumeTrue;
 
 import com.google.api.gax.rpc.NotFoundException;
 import com.google.cloud.firestore.BulkWriter;
@@ -263,12 +264,16 @@ private boolean dfsContainsCallStack(long spanId, List<String> expectedCallStack
   // Required to set custom-root span
   private static OpenTelemetrySdk openTelemetrySdk;
 
+  private static boolean isNightlyTesting;
   private static String projectId;
 
   private static Firestore firestore;
 
   @BeforeClass
   public static void setup() throws IOException {
+    String jobType = System.getenv("GITHUB_ENV_VAR_KOKORO_JOB_TYPE");
+    isNightlyTesting = jobType != null && jobType.equalsIgnoreCase("nightly");
+
     projectId = FirestoreOptions.getDefaultProjectId();
     logger.info("projectId:" + projectId);
 
@@ -361,6 +366,9 @@ public void before() throws Exception {
     customSpanContext = getNewSpanContext();
     assertNotNull(customSpanContext);
     assertNull(retrievedTrace);
+
+    // We only perform end-to-end tracing tests on a nightly basis.
+    assumeTrue(isNightlyTesting);
   }
 
   @After