test(spanner): unflake BuiltInMetricsCustomOpenTelemetryTest and BuiltInMetricsBothSinksTest (#14282)

In `executeSelect1()`, `resultSet.next()` was called only once for
`SELECT 1`. Closing the `ResultSet` before consuming the stream to EOF
invokes `iterator.close()`, which sends `call.cancel("ResultSet closed",
null)` to the underlying gRPC stream.

If client cancellation races ahead of the mock server's `onCompleted()`,
the stream terminates with `Status.CANCELLED`. As a result,
`SpannerTracer.operationSucceeded()` is never invoked, omitting
`Spanner.ExecuteStreamingSql` (status `OK`) from
`spanner.googleapis.com/internal/client/operation_count` and causing the
test assertion to fail.

Consume the `ResultSet` to EOF (`assertFalse(resultSet.next())`) in both
`BuiltInMetricsCustomOpenTelemetryTest` and
`BuiltInMetricsBothSinksTest` to guarantee that the stream finishes
normally before closing.

Author: sakthivelmanii

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

@@ -102,6 +102,7 @@ private static void executeSelect1(Spanner spanner) {
     try (ResultSet resultSet = client.singleUse().executeQuery(SELECT1)) {
       assertThat(resultSet.next()).isTrue();
       assertThat(resultSet.getLong(0)).isEqualTo(1L);
+      assertThat(resultSet.next()).isFalse();
     }
   }
 

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

@@ -17,6 +17,7 @@
 package com.google.cloud.spanner;
 
 import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
@@ -79,6 +80,7 @@ private static void executeSelect1(Spanner spanner) {
     try (ResultSet resultSet = client.singleUse().executeQuery(SELECT1)) {
       assertTrue(resultSet.next());
       assertThat(resultSet.getLong(0)).isEqualTo(1L);
+      assertFalse(resultSet.next());
     }
   }