docs: add Samples section to CONTRIBUTING.rst

[gcf-owl-bot]

Source-Link: googleapis/synthtool@52e4e46
Post-Processor: gcr.io/repo-automation-bots/owlbot-python:latest@sha256:6186535cbdbf6b9fe61f00294929221d060634dae4a0795c1cefdbc995b2d605

[tseaver]

@parthea One note about this batched set of PRs: the prefix should be docs:, not feat: -- there is nothing in the behavior of the library which changed at all.

[gcf-merge-on-green]

Merge-on-green attempted to merge your PR for 6 hours, but it was not mergeable because either one of your required status checks failed, one of your required reviews was not approved, or there is a do not merge label. Learn more about your required status checks here: https://help.github.com/en/github/administering-a-repository/enabling-required-status-checks. You can remove and reapply the label to re-run the bot.

[tseaver]

Unit test failures on Python 3.7, 3.8, and 3.9 are all in opentelemetry-related assertions, likely due to a recent release of opentelemetry-instrumentation 0.23b0. Python 3.6 tests pass, because testing/constraints-3.6.txt pins the minimum version (0.20b0). The following was sufficient to get tests passing again for me locally:

$ .nox/unit-3-9/bin/pip install --upgrade opentelemetry-{instrumentation,semantic-conventions}==0.22b0 opentelemetry-{api,sdk}==1.3.0

[parthea]

@larkee , please can you review/approve https://github.com/googleapis/python-spanner/pull/366/files first? Once it's merged, I can pull in those changes into this PR to avoid conflicts with owlbot.py

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[parthea]

@googlebot I consent.

[parthea]

Superseded by #426