fix(spanner): call ctx.cancel after stats have been recorded (googlea…

…pis#2728)

The cancel() method should be called once all code that is executing in the given
context have finished. In this case, that also means after the stats recording that
uses the same context has finished.

Fixes googleapis#2660

spanner/session.go

@@ -1501,7 +1501,6 @@ func (hc *healthChecker) worker(i int) {
 		if ws != nil {
 			ctx, cancel := context.WithTimeout(context.Background(), time.Minute)
 			err := ws.prepareForWrite(ctx)
-			cancel()
 			if err != nil {
 				// Skip handling prepare error, session can be prepared in next
 				// cycle.
@@ -1516,6 +1515,7 @@ func (hc *healthChecker) worker(i int) {
 			hc.pool.mu.Lock()
 			hc.pool.decNumBeingPreparedLocked(ctx)
 			hc.pool.mu.Unlock()
+			cancel()
 			hc.markDone(ws)
 		}
 		rs := getNextForPing()