chore: release spanner 1.20.0 (#4230)

diff --git a/spanner/CHANGES.md b/spanner/CHANGES.md
index 00f2174..c54c491 100644
--- a/spanner/CHANGES.md
+++ b/spanner/CHANGES.md
@@ -1,5 +1,12 @@
 # Changes
 
+## [1.20.0](https://www.github.com/googleapis/google-cloud-go/compare/spanner/v1.19.0...spanner/v1.20.0) (2021-06-08)
+
+
+### Features
+
+* **spanner:** add the support of optimizer statistics package ([#2717](https://www.github.com/googleapis/google-cloud-go/issues/2717)) ([29c7247](https://www.github.com/googleapis/google-cloud-go/commit/29c724771f0b19849c76e62d4bc8e9342922bf75))
+
 ## [1.19.0](https://www.github.com/googleapis/google-cloud-go/compare/spanner/v1.18.0...spanner/v1.19.0) (2021-06-03)