chore: Add Cloud Spanner OC client metrics sample

[c24t]

Description

This PR addresses #6514, adds a sample for capturing Cloud Spanner gRPC metrics with OpenCensus.

Note that we need to do extra work to instrument gRPC to emit the stats we're collecting here. We may want to wait to merge this PR or include this in public docs until we make the required gRPC changes.

FYI @larkee you're the owner of /spanner/**/*.py in CODEOWNERS, let me know if you want me to add a separate entry for OC samples and make myself the owner.

cc @mayurkale22

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.6 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• Please merge this PR for me once it is approved.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• spanner/opencensus/capture_grpc_metric.py:21, tag spanner_opencensus_capture_grpc_metric

---

This comment is generated by snippet-bot.
If you find problems with this result, please file an issue at:
https://github.com/googleapis/repo-automation-bots/issues.
To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment

[larkee]

LGTM other than maybe missing tests.

However, given that the Spanner samples have been moved to googleapis/python-spanner I'm wondering if this is the right place for this sample.

[dinagraves]

Hi Chris! Could you move this sample to the python-spanner repo? It also might be a good idea to put a placeholder here to let people know where to look. See example here.

[c24t]

Closing in favor of googleapis/python-spanner#568.