fix: blanks span for session keepAlive traces (#797)

The keepAlive traces were being tracked along with the parent span set
by the client, which clutters the tracing stack. Here, we set the
tracing to blank before issueing the keepAlive query.

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionImpl.java

@@ -113,6 +113,10 @@ void setCurrentSpan(Span span) {
     currentSpan = span;
   }
 
+  Span getCurrentSpan() {
+    return currentSpan;
+  }
+
   @Override
   public long executePartitionedUpdate(Statement stmt, UpdateOption... options) {
     setActive(null);

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionPool.java

@@ -77,6 +77,7 @@
 import io.opencensus.metrics.Metrics;
 import io.opencensus.trace.Annotation;
 import io.opencensus.trace.AttributeValue;
+import io.opencensus.trace.BlankSpan;
 import io.opencensus.trace.Span;
 import io.opencensus.trace.Status;
 import io.opencensus.trace.Tracer;
@@ -1472,11 +1473,15 @@ public void prepareReadWriteTransaction() {
 
     private void keepAlive() {
       markUsed();
+      final Span previousSpan = delegate.getCurrentSpan();
+      delegate.setCurrentSpan(BlankSpan.INSTANCE);
       try (ResultSet resultSet =
           delegate
               .singleUse(TimestampBound.ofMaxStaleness(60, TimeUnit.SECONDS))
               .executeQuery(Statement.newBuilder("SELECT 1").build())) {
         resultSet.next();
+      } finally {
+        delegate.setCurrentSpan(previousSpan);
       }
     }