chore: release spanner 1.12.0
# Changes
+## [1.12.0](https://www.github.com/googleapis/google-cloud-go/compare/spanner/v1.11.0...v1.12.0) (2020-11-10)
+
+
+### Features
+
+* **spanner:** add metadata to RowIterator ([#3050](https://www.github.com/googleapis/google-cloud-go/issues/3050)) ([9a2289c](https://www.github.com/googleapis/google-cloud-go/commit/9a2289c3a38492bc2e84e0f4000c68a8718f5c11)), closes [#1805](https://www.github.com/googleapis/google-cloud-go/issues/1805)
+* **spanner:** export ToSpannerError ([#3133](https://www.github.com/googleapis/google-cloud-go/issues/3133)) ([b951d8b](https://www.github.com/googleapis/google-cloud-go/commit/b951d8bd194b76da0a8bf2ce7cf85b546d2e051c)), closes [#3122](https://www.github.com/googleapis/google-cloud-go/issues/3122)
+* **spanner:** support rw-transaction with options ([#3058](https://www.github.com/googleapis/google-cloud-go/issues/3058)) ([5130694](https://www.github.com/googleapis/google-cloud-go/commit/51306948eef9d26cff70453efc3eb500ddef9117))
+* **spanner/spannertest:** make SELECT list aliases visible to ORDER BY ([#3054](https://www.github.com/googleapis/google-cloud-go/issues/3054)) ([7d2d83e](https://www.github.com/googleapis/google-cloud-go/commit/7d2d83ee1cce58d4014d5570bc599bcef1ed9c22)), closes [#3043](https://www.github.com/googleapis/google-cloud-go/issues/3043)
+
## v1.11.0
* Features:
