This PR modifies existing samples and adds a sample to show how to use the PITR feature.

* feat: add sample for commit stats

* fix: use correct kwarg

* fix: correct super call

* fix: add missing super init call

* fix: update super init call

* fix: use correct key

* refactor: remove testing file

* test: fix typo

* Apply suggestions from code review

Co-authored-by: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
Co-authored-by: skuruppu <skuruppu@google.com>

* refactor: make last_commit_stats public

Co-authored-by: larkee <larkee@users.noreply.github.com>
Co-authored-by: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
Co-authored-by: skuruppu <skuruppu@google.com>

* feat: add `Database.list_tables` method

* update docs, add tests

* remove numeric from get_schema test

The NUMERIC column is not included in the emulator system tests.

* add unit tests

* add docs for table api and usage

* fix link to Field class

* typo in table constructor docs

* add reload and exists methods

* feat: add table method to database

* update usage docs to use factory method

* address warning in GitHub UI for sphinx header

* Update docs/table-usage.rst

Co-authored-by: larkee <31196561+larkee@users.noreply.github.com>

Co-authored-by: larkee <larkee@users.noreply.github.com>

* samples: creates a backup using the current time

Instead of using the earliest version time of the database, uses the
current time (from spanner). If we used the earliest version time of the
database instead we would be creating an empty backup, since the
database we are backing up is not 1 hour old (default version retention
period).

* samples: addresses pr comments

Refactors create backup sample

🤖 I have created a release \*beep\* \*boop\* 
---
## [3.2.0](https://www.github.com/googleapis/python-spanner/compare/v3.1.0...v3.2.0) (2021-03-02)


### Features

* add `Database.list_tables` method ([#219](https://www.github.com/googleapis/python-spanner/issues/219)) ([28bde8c](https://www.github.com/googleapis/python-spanner/commit/28bde8c18fd76b25ec1b64c44db7c1600255256f))
* add sample for commit stats ([#241](https://www.github.com/googleapis/python-spanner/issues/241)) ([1343656](https://www.github.com/googleapis/python-spanner/commit/1343656ad43dbc41c119b652d8fe9360fa2b0e78))
* add samples for PITR ([#222](https://www.github.com/googleapis/python-spanner/issues/222)) ([da146b7](https://www.github.com/googleapis/python-spanner/commit/da146b7a5d1d2ab6795c53301656d39e5594962f))


### Bug Fixes

* remove print statement ([#245](https://www.github.com/googleapis/python-spanner/issues/245)) ([1c2a64f](https://www.github.com/googleapis/python-spanner/commit/1c2a64fd06404bb7c2dfb4a8f65edd64c7710340))
---


This PR was generated with [Release Please](https://github.com/googleapis/release-please). See [documentation](https://github.com/googleapis/release-please#release-please).