Spanner: ResultSet from session keep-alive request is not closed.

[sebright]

When I ran an application using google-cloud-spanner 1.30.0 with opencensus-impl 0.23.0, I noticed this log message:

io.opencensus.implcore.trace.RecordEventsSpanImpl finalize
SEVERE: Span CloudSpannerOperation.ExecuteStreamingQuery is GC'ed without being ended.

It looks like this is caused by the session keep-alive request:

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

Lines 790 to 796 in ca141fc

	private void keepAlive() {
	markUsed();
	delegate
	.singleUse(TimestampBound.ofMaxStaleness(60, TimeUnit.SECONDS))
	.executeQuery(Statement.newBuilder("SELECT 1").build())
	.next();
	}

If I understand correctly, the ResultSet contains a ResumableStreamIterator, which contains the span representing the request. The issue is that keepAlive doesn't call close on the ResultSet, so the span is never ended. Calling close on the ResultSet seems to fix the issue.